If you've ever wondered how your iPhone's Calendar app talks to your work calendar server, or how Thunderbird syncs with Fastmail, or how a Nextcloud instance shares events with macOS — the answer, almost certainly, is CalDAV. RFC 4791 is the protocol that quietly powers most non-Google, non-Microsoft calendaring on Earth.

The problem. By the mid-2000s, calendar data had a standard format (iCalendar, RFC 5545) but no standard way to share it. Every vendor had its own proprietary server protocol: Exchange used MAPI/RPC, Lotus Notes had NRPC, Oracle had its own thing. Open-source clients couldn't interoperate, and users couldn't move between platforms without losing their data. What was needed was an open, HTTP-based protocol that any client could speak.

The clever design decision: build on WebDAV. Rather than invent a new protocol from scratch, the authors layered calendaring on top of WebDAV (RFC 4918) — HTTP's extension for collaborative file management. A calendar becomes a collection (think: directory), and each event or task is a resource (think: file) containing an iCalendar object. To create an event, you PUT an .ics file. To delete it, you DELETE it. To list events, you use WebDAV's PROPFIND. This means CalDAV inherits HTTP authentication, TLS, ETags for conflict detection, and all the existing tooling for free.

The novel parts. CalDAV adds a few essential pieces WebDAV alone can't provide:

• The MKCALENDAR method — a new HTTP verb to create a calendar collection (distinct from a generic WebDAV collection).
• The REPORT method extensions: calendar-query lets clients ask "give me all VEVENTs between July 1 and July 31 that match these filters" without downloading the entire calendar. calendar-multiget fetches a known set of resources in one round trip.
• Server-side expansion of recurring events — the server can expand a weekly meeting into individual instances within a date range, so dumb clients don't have to implement RFC 5545's gnarly recurrence rules.
• Free-busy queries via VFREEBUSY, enabling "find a time" features across users.

Why it matters today. Apple bet heavily on CalDAV — macOS Calendar, iOS Calendar, and iCloud all speak it natively. Google supported it until 2024 when they restricted it to Workspace accounts, but the protocol still underpins essentially every non-Microsoft, non-Google calendar service: Fastmail, Nextcloud, Radicale, SOGo, Baïkal, Posteo, Mailbox.org, and self-hosted setups worldwide. Its sibling CardDAV (RFC 6352) does the same for contacts. If you own your data and aren't paying Microsoft or Google, you're almost certainly using CalDAV.

The quirks. The XML payloads are verbose — a simple query is hundreds of bytes of namespaced XML. The recurrence expansion rules in RFC 5545 are notoriously hard to get right, and CalDAV servers have historically disagreed on edge cases (timezones around DST transitions, EXDATEs on infinite recurrences). And because every event is a separate HTTP resource, syncing a large calendar can mean thousands of requests — though the sync-collection REPORT (RFC 6578) later helped enormously.

The history. Lisa Dusseault was a key WebDAV editor at the IETF; Cyrus Daboo went on to author the CardDAV spec and now works at Apple on calendaring; Bernard Desruisseaux came from Oracle, which had a major calendaring product. The collaboration across vendor lines is part of why CalDAV won where proprietary protocols stalled.