Etar Calendar

Etar Calendar is an open source calendar app for Android. It is built to show local and synchronized calendars in one place, with support for offline use and a simple way to manage events on the phone.

It includes month, week, day, and agenda views, plus dark and light themes. It uses Android calendar storage, can show all synchronized calendars, and lets you choose which calendars appear and which one receives a new event. It also supports calendar sharing via ICS and import and export from SD card.

Etar is an enhanced version of AOSP Calendar and is licensed under GPLv3. It ships through F-Droid and Google Play. For syncing to a server, it relies on another CalDAV app such as DAVx5 rather than including a CalDAV client itself.