Alternatives to Google Calendar

Works on MicroG?

The client is not working. You can sync with the All In One module in MicroG Setup module section, or you can use DavX5, look ahead.

Open Source Offline

Simple Calendar Pro

Simple Calendar Pro is a fully customizable, offline calendar designed to do exactly what a calendar should do. No complicated features, unnecessary permissions and no ads!

AOSP/Lineage Calendar

Simple and reliable. Already included in many custom roms

Etar

Etar is a material designed open source calendar

Open Source Sync

DAVx⁵

DAVx⁵ is a CalDAV/CardDAV management and synchronization app for Android which natively integrates with Android calendar/contact apps.

How to sync with Google Contacts and Calendar

Closed Source Sync

Not yet