Open-source material design Calendar that integrates with the Android Calendar API. If you have a CalDAV/CardDAV server, I recommend using [DAVx5](https://www.davx5.com/) to sync with it. Available from F-Droid. Also look at [ICSx5](https://icsx5.bitfire.at/), another WebDav/CalDav client that can integrate with calendar apps. Also available from F-Droid. Thanks @je-vv for ICSx5.