When adding a Thundermail account to Thunderbird or other email clients, that client will prompt you to add your account’s calendar data, which is handled by an Internet protocol called CalDAV (Calendaring Extensions to Distributed Authoring and Versioning).
Through CalDAV, you will be able to connect as many clients as you wish to sync with all of your personal and shared Thundermail calendar events.
If your client does not automatically import your calendar information when you add your Thundermail account, you can enter your CalDAV URL manually into its settings.
That URL is formatted like this:
https://mail.thundermail.com/dav/cal/<userid>%40thundermail.com/default/
For example, if your Thundermail address is example@thundermail.com, your CalDAV URL is:
https://mail.thundermail.com/dav/cal/example%40thundermail.com/default/