Best Open-Source Calendar Apps 2026: 6 Options Compared

Open-source calendars come in three shapes, and mixing them up leads to frustration. A calendar server, such as Radicale or Baikal, stores events and speaks CalDAV but has almost no interface, so you pair it with a client. A client, such as Thunderbird, GNOME Calendar or Etar on Android, shows your events and syncs with a server or with a provider you already use. A suite, such as Nextcloud, bundles the server, a web calendar and file storage, plus contacts and tasks. If you only want your own events off Google's servers, run Radicale or Baikal on a small machine and point Thunderbird and Etar at it. If you share calendars with a family or a team, Nextcloud is the heavier but friendlier choice because people get a web page instead of a protocol. The trade-off is upkeep: any server you host needs HTTPS, backups and updates, and a broken calendar is noticed immediately. Android users have fewer good choices on iOS, where Apple's built-in app talks CalDAV anyway. Licences below come from each project's repository, and none of these projects was run for this guide, so test sync with a throwaway calendar first.

Thunderbird

Free, MPL-2.0 desktop email client that also gives you a calendar on Windows, macOS and Linux; the project page calls it a free email application. Pair it with a CalDAV server.

Price: Free (Free and open-source desktop mail client; no paid plan stated on the about page) (Thunderbird pricing, checked 2026-10-23).

Platforms and license: Windows, macOS, Linux. MPL-2.0 (open source) (license source).

Best for: Desktop users who also want email.

Nextcloud

AGPL-3.0 self-hosted suite whose Calendar app supports creating appointment configurations people can book, alongside files and contacts. Community server is free to host.

Price: Free (Community server is free to self-host and the Nextcloud-branded desktop and mobile apps are free); paid from EUR 71.29 per user/year (Enterprise Standard at 100 users, 100-user minimum) (Nextcloud pricing, checked 2026-10-22).

Platforms and license: Web, Self-hosted, Desktop app, Mobile app. AGPL-3.0 (open source) (license source).

Project activity: 37.3k GitHub stars · latest release v33.0.10 (Oct 15, 2026) · last commit Oct 27, 2026

Best for: Families and teams sharing calendars.

Radicale

Small GPL-3.0 CalDAV and CardDAV server for calendars, to-do lists and contacts. No heavy interface, which keeps it light but means you need a separate client.

Price: Free (Free and open source CalDAV and CardDAV server) (Radicale pricing, checked 2026-10-24).

Platforms and license: Self-hosted. GPL-3.0 (open source) (license source).

Project activity: 5.2k GitHub stars · latest release v3.9.0 (Oct 25, 2026) · last commit Oct 27, 2026

Best for: Minimal self-hosted sync.

Worth knowing: README: small CalDAV (calendars, to-do lists) and CardDAV (contacts) server. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

Baikal

GPL-3.0 CalDAV and CardDAV server from the sabre.io team. A good fit if you want a simple web admin for a few users instead of editing config files.

Price: Free (Free and open source CalDAV and CardDAV server) (Baikal pricing, checked 2026-10-24).

Platforms and license: Self-hosted. GPL-3.0 (open source) (license source).

Project activity: 3.3k GitHub stars · latest release 0.13.0 (Oct 19, 2026) · last commit Oct 19, 2026

Best for: Small server with web admin.

Worth knowing: License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

Etar Calendar

GPL-3.0 Android calendar with a material design that syncs through whatever accounts your phone has. It is a client, not a server.

Price: Free (Free and open source Android calendar) (Etar Calendar pricing, checked 2026-10-24).

Platforms and license: Android. GPL-3.0 (open source) (license source).

Project activity: 2.6k GitHub stars · latest release v1.0.57 (Jul 27, 2026) · last commit Oct 26, 2026

Best for: Android phones.

Worth knowing: README: open source, material-designed calendar. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

GNOME Calendar

GPL-3.0 calendar for the GNOME desktop that shows events from your online accounts. It is a client, so it needs an account or server behind it.

Price: Free (Free and open source desktop calendar) (GNOME Calendar pricing, checked 2026-10-24).

Platforms and license: Desktop (GNOME). GPL-3.0 (open source) (license source).

Project activity: 116 GitHub stars · last commit Oct 27, 2026

Best for: Linux GNOME desktops.

Worth knowing: GitHub repository is a read-only mirror of the GNOME GitLab project. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

Which one to pick

• You want your events off Google but have one small spare server: Radicale
• You share a calendar with a family or small team: Nextcloud
• You use Android and want a clean offline-capable view: Etar Calendar
• You read mail and manage events in one desktop app: Thunderbird