Feature spec
IntermediateCategory
vertical-tools
Build with AI
3–6 hours with Lovable or V0
Custom build
1–2 weeks custom dev
Running cost
$0–$30/mo up to 1K users
Works on
Everything it takes to ship an Event Calendar — parts, prompts, and real costs.
An event calendar needs a grid UI (FullCalendar.js), a Supabase events table with recurrence rules, and an RSVP Edge Function that enforces capacity server-side. With Lovable or V0 you can ship a working calendar in 3–6 hours for $0–$30/month. Costs start only when Supabase Pro is needed at around 1,000 users or when RSVP email volume exceeds Resend's free tier.
What an Event Calendar Feature Actually Is
An event calendar lets visitors browse, filter, and register for upcoming events directly inside your app — no third-party ticketing link required. The feature covers three layers: a grid UI that renders month, week, and day views; a database of events with recurrence rules (so a weekly standup does not require 52 manual entries); and an RSVP engine that enforces seat capacity and sends confirmation emails. The product decisions that matter are: who can create events (organizers only, or anyone?), how recurring events expand without blowing up your database, and whether RSVPs are anonymous or authenticated. Getting those three right on the first build is the difference between a calendar that impresses and one that gets replaced.
What users consider table stakes in 2026
- Month/week/day view switching with a visible toggle — users expect to narrow from a broad month overview to a specific day's slots
- Click-to-see-detail popover or modal with event description, organizer name, capacity remaining, and a one-click RSVP button
- Color-coded event categories so users can visually filter workshops from meetups from webinars at a glance
- Recurring events displayed correctly on every applicable date without UI duplication artifacts or stale data
- Timezone-aware display that shows event times in the viewer's local timezone, not the organizer's
- Mobile-responsive grid that collapses from a 7-column month view to a vertical day list below 768px
Anatomy of the Feature
Five components, two of which AI tools consistently get wrong without explicit prompting: the recurrence engine and the RSVP capacity check.
Calendar grid UI
UIFullCalendar.js (open-source, 18K GitHub stars) renders month, week, and day views with built-in drag-and-drop event editing. A shadcn/ui Dialog wraps the event detail popover when a user clicks an event tile.
Note: FullCalendar is browser-only — it reads window and document globals on import. Wrapping in next/dynamic with ssr:false is mandatory in any Next.js app.
Event data layer
DataA Supabase table named events stores id, title, start_at, end_at, category, description, recurrence_rule, max_capacity, and created_by. A Supabase Realtime subscription keeps the calendar live when an organizer edits an event while viewers are on the page.
Note: Store start_at and end_at as timestamptz in UTC. Displaying in the viewer's local timezone is a frontend concern handled by date-fns-tz.
Recurrence engine
Backendrrule.js parses RFC 5545 RRULE strings (e.g. FREQ=WEEKLY;BYDAY=MO,WE,FR) stored in the database and expands them into individual occurrences on the frontend at render time.
Note: Never expand recurring events into individual DB rows on save — that creates hundreds of orphaned rows when the organizer edits the rule. Expand at render time using event_id + occurrence_date as the composite key to prevent re-render duplication.
RSVP and registration engine
BackendA Supabase Edge Function handles RSVP writes: it checks current RSVP count against max_capacity inside a transaction before inserting into the rsvps table, then calls Resend to send a confirmation email to the attendee.
Note: Client-side capacity checks are exploitable — two users can pass the check simultaneously before either INSERT commits. The Edge Function is the only safe place for this logic.
Auth gate
ServiceSupabase Auth (email magic link or OAuth) differentiates organizers from viewers. Row-level security policies restrict INSERT, UPDATE, and DELETE on the events table to the row's created_by user, while SELECT is open to all.
Note: OAuth login (Google, GitHub) fails in the Lovable preview iframe due to sandboxed third-party cookie restrictions. Test RSVP flows on the published URL.
The data model
Two tables cover events and RSVPs with row-level security. Run this in the Supabase SQL editor — it is copy-paste ready.
1create table public.events (2 id uuid primary key default gen_random_uuid(),3 title text not null,4 start_at timestamptz not null,5 end_at timestamptz not null,6 category text not null default 'general',7 description text,8 recurrence_rule text,9 max_capacity int,10 created_by uuid references auth.users(id) on delete cascade not null,11 created_at timestamptz not null default now()12);1314alter table public.events enable row level security;1516create policy "Events are viewable by everyone"17 on public.events for select18 using (true);1920create policy "Organizers can insert their own events"21 on public.events for insert22 with check (auth.uid() = created_by);2324create policy "Organizers can update their own events"25 on public.events for update26 using (auth.uid() = created_by);2728create policy "Organizers can delete their own events"29 on public.events for delete30 using (auth.uid() = created_by);3132create index events_start_at_idx on public.events (start_at);33create index events_created_by_idx on public.events (created_by);3435create table public.rsvps (36 id uuid primary key default gen_random_uuid(),37 event_id uuid references public.events(id) on delete cascade not null,38 user_id uuid references auth.users(id) on delete cascade not null,39 created_at timestamptz not null default now(),40 unique (event_id, user_id)41);4243alter table public.rsvps enable row level security;4445create policy "Users can view own RSVPs"46 on public.rsvps for select47 using (auth.uid() = user_id);4849create policy "Users can RSVP to events"50 on public.rsvps for insert51 with check (auth.uid() = user_id);5253create policy "Users can cancel own RSVPs"54 on public.rsvps for delete55 using (auth.uid() = user_id);5657create index rsvps_event_id_idx on public.rsvps (event_id);58create index rsvps_user_id_idx on public.rsvps (user_id);Heads up: The unique constraint on (event_id, user_id) prevents one user from RSVPing twice. Total capacity overflow is prevented in the Edge Function — the DB constraint alone does not protect against two different users simultaneously exceeding capacity.
Build it — pick your path
Each tool fits this feature differently. Switch tabs to compare fit, time, the exact steps, and a copy-paste prompt.
Recommended when the calendar is the core product feature. Custom development adds iCal/Google Calendar two-way sync, multi-timezone organizer support, paid RSVP via Stripe, and white-labeling for multiple clients.
Step by step
- 1Full FullCalendar.js integration with custom recurrence UI and iCal .ics file export for each event
- 2Google Calendar API two-way sync: import organizer's Google Calendar events and push confirmed RSVPs back to attendee Google Calendar
- 3Stripe Payment Element at RSVP time for paid events — webhooks finalize RSVP only after payment confirmed
- 4Multi-organizer permission model with approval workflows for community-submitted events
Where this path bites
- Two-way Google Calendar sync requires OAuth consent screen review — Google review takes 4–6 weeks for sensitive scopes
- Paid RSVP via Stripe adds refund handling complexity and Stripe account verification requirements
Third-party services you'll need
Three services power the full-featured calendar. Supabase tier is the main cost lever — the calendar grid itself adds no service cost.
| Service | What it does | Free tier | Paid from |
|---|---|---|---|
| Resend | Transactional email for RSVP confirmations and event reminder emails sent 24h before the event | 3,000 emails/month | $20/mo for 50K emails (approx) |
| Google Calendar API | Optional: sync events to/from Google Calendar so attendees can add events with one tap from their RSVP confirmation | Free for read; push notifications quota-limited but $0 per call | Free (quota applies) |
| Supabase | PostgreSQL database, Auth for organizer/attendee roles, Realtime subscriptions for live event updates, Edge Functions for RSVP capacity enforcement | 2 projects, 500MB database | $25/mo Pro plan |
Swipe the table sideways to see pricing.
What it costs to run
Drag through the tiers to see how your monthly bill scales with users — no surprises later.
Estimated monthly running cost
Supabase free tier handles the events table and RSVP history at this scale. Resend free tier covers all confirmation emails at low RSVP volume. No paid calendar service required.
Estimates use verified 2026 pricing and assume typical usage per user. Your real bill depends on activity, storage, and third-party plans.
What breaks when AI tools build this
The failures people actually hit on their first build — the symptom, why it happens, and the exact fix.
FullCalendar crashes Next.js with 'window is not defined'
Symptom: FullCalendar reads browser globals (window, document, navigator) on import. When Next.js renders a Server Component or runs static generation, there is no browser environment — the import throws immediately and breaks the build or causes a runtime error on the server.
Fix: Wrap the FullCalendar import in next/dynamic with { ssr: false }. This defers the import to the browser only. Apply this to any component file that directly imports from @fullcalendar/react or any @fullcalendar/* package.
Recurring events duplicate on every re-render
Symptom: AI tools commonly expand recurring events by running rrule.js inside a useEffect that re-fires on every state change, generating new Date objects each time. React's reconciliation treats these as new items, causing the calendar to remount event tiles and show duplicate counts.
Fix: Expand occurrences once using useMemo, keying each expanded event by event_id + ISO date string (e.g. 'evt_abc123_2026-07-14'). FullCalendar uses the event id field for stable rendering — always set it to the composite key.
RSVP race condition lets capacity overflow
Symptom: The most common AI-generated RSVP flow is: fetch RSVP count client-side, check against max_capacity in React, then call INSERT if capacity allows. Two users submitting RSVP simultaneously both pass the client-side check before either row commits to the database.
Fix: Move the capacity check into a Supabase Edge Function that runs SELECT COUNT then INSERT inside a single transaction. Return a 409 Conflict error to the second user. Alternatively, enforce a CHECK constraint in Postgres combined with an application-level retry, though the Edge Function gives a cleaner error message to the user.
Events display at wrong time for international users
Symptom: When datetimes are stored as plain text strings or as timestamps without timezone offset, and displayed as-is, a 2:00 PM New York event appears as 2:00 PM to a user in London — 5 hours off. This is the single most common complaint in community event apps after launch.
Fix: Always store start_at and end_at as timestamptz in UTC. On the frontend, use date-fns-tz to convert to the viewer's local timezone: utcToZonedTime(event.start_at, Intl.DateTimeFormat().resolvedOptions().timeZone).
OAuth login for RSVP fails inside Lovable preview
Symptom: The Lovable preview runs in a sandboxed iframe that blocks third-party cookies and cross-origin popups. When users click 'RSVP with Google', the OAuth popup is blocked or the redirect fails to return to the correct origin, leaving the user stuck at the provider's login page.
Fix: Test all RSVP and auth flows exclusively on the published Lovable URL, not in the preview. The preview is safe for layout and visual QA only — authentication requires the published HTTPS environment.
Best practices
Always expand recurring events at render time using rrule.js — never write individual rows to the database for each occurrence, or you will have hundreds of orphaned rows when an organizer edits the rule
Enforce RSVP capacity server-side in a Supabase Edge Function — a race condition at capacity limit is a support nightmare that erodes trust
Store all event times as UTC timestamptz and convert to the viewer's local timezone on the frontend using date-fns-tz
Wrap FullCalendar in next/dynamic with ssr:false in any Next.js project — it is a browser-only library and will break SSR builds without this guard
Color-code event categories with a fixed palette defined in the database — give organizers a predefined set of colors, not a free-form picker, for visual consistency
Add a Supabase Realtime subscription so organizers can edit events while viewers see updates instantly without a page refresh
Grey out past events rather than hiding them — users frequently look back to reference past events, organizers, and attendance before deciding to RSVP to the next edition
Send RSVP confirmation emails with an .ics attachment so attendees can add the event to their personal calendar with one click from any email client
When You Need Custom Development
Lovable and V0 handle a solid community events calendar well. These are the signs the project has outgrown AI-assisted builds:
- The calendar needs two-way Google Calendar sync — import the organizer's existing calendar and push confirmed RSVPs back to attendee calendars (requires Google OAuth consent screen review, which takes 4–6 weeks)
- Events require paid RSVPs or deposits via Stripe, with automatic refund handling when organizers cancel — the Stripe webhook-to-RSVP-status flow needs careful sequencing
- Multiple organizers share one calendar with an approval workflow — submitted events are reviewed by an admin before going live, with role-based permissions at the database level
- The calendar must be white-labeled and embedded in multiple client sites, each with their own event set and branding
RapidDev builds these features for production
Real auth, database, and payments — fixed price, delivered in weeks. You own 100% of the code.
Frequently asked questions
Can I add a calendar without a database?
For a purely read-only calendar with hardcoded or CMS-managed events, yes — FullCalendar renders from a static JSON array. The moment you add RSVPs, capacity limits, recurring events with edit support, or organizer accounts, you need a database. Supabase free tier handles all of this at zero cost until you exceed 500MB of data.
How do I handle recurring events?
Store the RRULE string in a single database row (e.g. FREQ=WEEKLY;BYDAY=MO). At render time, use rrule.js on the frontend to expand it into individual occurrence dates and feed them to FullCalendar. This way editing the recurrence rule updates all future occurrences in one database write, not hundreds of individual rows.
Can users add events to Google Calendar from mine?
Yes, with two approaches: the simple path is generating an .ics file that any calendar app opens — Resend can attach it to the RSVP confirmation email. The advanced path uses the Google Calendar API to add the event directly to the user's Google Calendar after OAuth consent. The .ics approach takes 30 minutes to implement; the Google Calendar API takes 2–4 hours and requires a Google Cloud project.
What's the difference between FullCalendar and a custom-built grid?
FullCalendar gives you month/week/day views, drag-and-drop editing, timezone support, and recurring event display out of the box — all free and open-source. A custom-built CSS grid takes 2–4 days to reach the same feature parity and typically has edge cases in month boundary rendering and leap year handling. Unless you need pixel-perfect design control or a completely non-standard layout, FullCalendar is the correct choice.
How do I prevent double-booking for limited-capacity events?
Add a unique constraint on (event_id, user_id) in the rsvps table to prevent one user RSVPing twice. For total capacity, enforce the check in a Supabase Edge Function that runs SELECT COUNT then INSERT in a transaction — never trust a client-side capacity check, as two simultaneous clicks can both pass before either commits to the database.
Can I show events filtered by category?
Yes. FullCalendar has built-in event source filtering. Store a category field in your events table, pass the active filter as a parameter to your Supabase query, and update FullCalendar's event source when the filter changes. A category pill filter bar above the calendar is a well-understood UX pattern that most users find immediately.
How do I send event reminder emails?
At RSVP time, use a Supabase Edge Function to calculate the time 24 hours before the event and schedule a Resend email. For higher reliability, use pg_cron (available on Supabase Pro) to run a nightly job that queries rsvps where event start_at is within 24 hours and sends batch Resend emails. The pg_cron approach is more reliable for high volume because it does not depend on a background task staying alive.
Can I embed this calendar in an iframe on another site?
Yes — publish the app and embed the /events URL in an iframe on your marketing site. Note that OAuth flows inside an iframe break for the same reason they break in the Lovable preview — users will need to authenticate in the main window before the iframe calendar picks up their session, or open the calendar in a new tab for registration.
Need this feature production-ready?
RapidDev builds an event calendar into real apps — auth, database, payments — at $13K–$25K.
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