Radicale

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.

Via F-droid install DAVdroid.

There seems to be a problem with Let's Encrypt certificates with this app, but it's possible to get around it. Open DAVdroid and select the side menu followed by Settings. Enable Distrust system certificates and press Reset untrusted certificates.

Exit from settings and press the plus button to add an account. Select Login with URL and user name. 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 Passwords section of the Administrator control panel.

You will be prompted to approve the Let's Encrypt cerificate for your domain name, and once that's done then you should see your account as a large yellow box. Press on that and ensure that Addresses and calendar are selected.

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.