Calendar is tricky

[Johannes]

Here’s an old topic that could potentially (as this was before Android 7) still be useful:

FP Open: No local Calendar and Calendar only syncs with Google

Expected behaviour: the ability to create a local (phone only) calendar without the need to use a potentially insecure shared online calendar. the ability to sync with other calendaring systems like DavDroid. Acutal behaviour: A warning “you have to create at least one calendar account” appears when creating an event with the default settings. It is not possible to create a local (phone only) calendar. Only Google Calendars work. Non Google Calendars (DavDroid) show up in the list “calendar…

Two solutions there are either an offline calendar account provider from f-droid or an online account that is capable of talking caldav.
In addition, there’s some suggestions for alternative apps here:

Can I Use The Calendar Without Email?

I guess you are using FPOS with Google services, thus your default (and only) calendar app is Google calendar, which is forcing you to have a Google account for using it, right? Common “don’t be evil” Google behaviour [/sarcasm] If Offline Calendar from F-Droid (which doesn’t require any account to fully work, by the way) doesn’t do the trick, you can try another Calendar app which doesn’t force you to register any account, like Standalone Calendar or the material designed Etar. PD: If you are…