Radicale
Radicale is a calendar server which allows your to synchronise your calendar across all your devices. Support for CalDAV within various systems seems rather patchy/flaky though, so use it with caution.
Installation
Log into your system with:
ssh myusername@mydomain -p 2222
Using cursor keys, space bar and Enter key select Administrator controls and type in your password.
Select Add/Remove Apps then radicale. If you don't already have an SSL/TLS certificate for your main domain then go to the security settings and create a new Let's Encrypt cert for it. That will ensure that your calendar events have some minimal level of protection from passive surveillance.
Setting up on Android
Via F-droid install DAVdroid. Within Android Settings select Accounts then DAVdroid. Select the URL option rather than the email option. The URL should be https://yourmaindomainname/radicale/. Remember to include the trailing slash on the URL. If you installed Freedombone from a disk image then enter your username and the password which was shown at the start of installation. If not then the password for Radicale will be within the README file within your home directory.
Now go to your calendar app and press the plus icon to add an event. You should notice that the calendar account selected is your username on the Freedombone system. Synchronisation should be automatic.