Open Source Calendar
A calendar is only as good as its sync, and sync is exactly where a free hosted calendar reads the most revealing log you keep - who you meet, where you go, when you travel. The open source options here split cleanly into two jobs: servers that speak CalDAV and iCalendar so any device can connect, and client apps that give you a nicer view onto a calendar that lives elsewhere. Knowing which one you need saves the classic mistake of installing a pretty client and then discovering it cannot solve sync, sharing, or invitations by itself. Either way, self-hosting keeps your schedule on infrastructure that syncs it without quietly studying it.
Leantime
Open source project management for non-project managers, built with ADHD, dyslexia, and autism in mind
Radicale
CalDAV and CardDAV server for calendars, to-do lists, contacts, and file-system storage
Baïkal
Self-hostable CalDAV and CardDAV server for syncing calendars and contacts with DAV clients
Etar Calendar
Open source material-designed calendar for Android with offline events and multiple views
SOGo
Open source groupware with webmail, calendars, address books, and standards-based client access
WeekToDo
Minimalist weekly planner and to do list app with local storage and reminders
Fossify Calendar
Private, ad-free calendar app with customizable widgets, syncing, and flexible event planning
Our picks
Servers, clients, and planners - pick by the role you need filled.
Best minimal CalDAV server: Radicale Radicale is a small, no-fuss CalDAV and CardDAV server that stores calendars, to-do lists, and contacts as plain files in a simple folder structure. It works out of the box with authentication and TLS, connects to many standard DAV clients, and extends through plugins. A good fit when you want to own sync without running heavy groupware.
Best for team calendaring: SOGo SOGo is full groupware with shared calendars, address books, and AJAX webmail, plus resource sharing and permission handling. Native clients connect through CalDAV, CardDAV, GroupDAV, and Microsoft ActiveSync, so iPhone, Android, Outlook, and Thunderbird can all sync events, contacts, and tasks. Choose it when a team needs delegated access and scheduling, not just personal sync.
Best Android calendar app: Fossify Calendar Fossify Calendar manages events without ads, tracking, or intrusive permissions, with daily through yearly views, reminders, advanced repetition rules, and home-screen widgets. It syncs with Google Calendar, Outlook, Nextcloud, and Exchange, so it can front a server you already run while keeping data on the device.
Best personal weekly planner: WeekToDo WeekToDo combines a lightweight calendar and to-do lists, letting you schedule tasks across the week with sub-tasks, recurring items, colors, and reminders. All data stays local on your device with no account required, on Windows, macOS, Linux, or the web. Note that cross-device sync and a mobile app are not available yet, so it suits single-machine planning.
Deciding between a calendar server and a calendar app
The first fork in the road is whether you need a server or just an app, and picking wrong is the usual reason a calendar migration stalls. Servers are where events actually live and sync: Radicale and Baikal are lightweight CalDAV and CardDAV servers, while SOGo is full groupware with webmail, shared calendars, and ActiveSync. Client apps are the interface onto a server you already run - Etar and Fossify display and edit calendars on Android but lean on a separate sync app or account. If you need shared calendars, device sync, and invitations, begin from a server; a nice client alone cannot provide them.
Whatever you choose, judge it on the protocol boundary. A calendar that speaks CalDAV and reads and writes iCalendar files gives you client choice and a clean exit; one built only around its own web view locks you in. The details that break in real use are recurring events, exceptions to a repeating series, time zones, alarms, and attendee replies, so test those rather than assuming basic event creation proves compatibility. Radicale, Baikal, and SOGo all target standard DAV clients, which is what lets iOS, macOS, Thunderbird, and Android clients connect to the same data.
Then match the permission model to your actual use. A personal planner like WeekToDo stores everything locally with no account, simple but offering no sharing or cross-device sync yet. A family needs shared calendars and subscriptions; a company needs delegated access, resource booking, and private-event visibility, where groupware like SOGo earns its weight. Confirm the exit too: bulk ICS export and per-user backup that a restore test actually verifies, since losing recurrence exceptions or attendees can hurt more than losing an event title.
Related categories
Frequently asked questions
Do I need a calendar server, or just a calendar app?+
Choose a server if you need shared calendars, device sync, invitations, and a stable place where events live - Radicale and Baikal handle CalDAV sync, and SOGo adds groupware on top. Choose an app if you already have a CalDAV backend and just want a better interface, which is what Etar and Fossify provide on Android. Many failed setups come from installing a nice client and only later discovering it cannot handle sync or access control alone.
How much does CalDAV support really matter?+
A lot, if you care about client choice and a clean exit. CalDAV is the common protocol that lets many desktop and mobile clients talk to one calendar server, and iCalendar is the file format events move in. Radicale, Baikal, and SOGo all speak it, so iOS, macOS, Thunderbird, and DAVx5 on Android can share the same data. Support quality still varies, so test recurring events, shared calendars, and delegated access rather than trusting a checkbox.
Will invitations and RSVPs work with people outside my system?+
They can, but calendar invites ride on email as much as on the calendar. Your server needs reliable outbound mail, a correct sender identity, and handling for replies, updates, and cancellations. SOGo is groupware built to route invitations, while a bare CalDAV server like Radicale focuses on sync rather than scheduling across organizations. Test with common external providers before rollout, since the event can store correctly while the email layer silently drops a reply.
How well do these sync to my phone?+
It depends on the app and the sync path. Etar shows synchronized calendars on Android but relies on a separate CalDAV app such as DAVx5 to talk to your server, while Fossify syncs directly with Google Calendar, Outlook, Nextcloud, and Exchange. Test creating events, accepting invites, editing recurring meetings, and receiving reminders on a flaky connection - mobile support is about more than viewing the schedule.
What breaks when I import my existing calendar?+
Basic imports through ICS files are usually straightforward, but fidelity depends on what the old system exports. One-off events survive well; recurring events with edited instances, attendee status, reminders, and shared-calendar ownership are the fragile parts. Before a cutover, import a copy into a test account and compare several months of real events rather than trusting a clean import report. Keep the export file until you have verified the result.
Which permissions matter for a shared team calendar?+
Look past read and write. Teams often need private events that show only busy time, delegated management for assistants, calendars owned by a group rather than a person, and clear rules for who can invite others - SOGo's groupware model covers resource sharing and permissions like these. If permissions are too coarse, people create workarounds such as duplicate calendars, which defeats the point of a shared system.
Can I use these without a Google or Microsoft account?+
Yes. Radicale, Baikal, and SOGo let you host the calendar yourself, so events sync through your own server instead of a big provider's account. On the client side, WeekToDo stores everything locally with no account at all, and Fossify runs on the device without a hosted service, though it can still sync to outside accounts if you choose. Self-hosting is what takes your schedule off infrastructure that could mine it.