Open Vivaldi, then follow the following steps:

1. Click on the Vivaldi calendar icon.
   Alternatively, you can open the calendar by going to Tools -> Calendar.
2. Click on the Calendars menu.
3. Select Calendar Settings.

4. In the Calendar Accounts section, below the list of accounts, click on the plus sign.
5. Select CalDAV.
6. Confirm your selection by clicking Continue.

7. In the Display Name field, enter a name of your choice - for example My Posteo Calendar.
8. In the Address field enter the URL below, substituting the youraddress part with the local-part of your Posteo email address. The local-part is everything that comes before the @ symbol:
   https://posteo.de:8443/calendars/youraddress/default
9. In the Username and Password fields, enter your Posteo email address and your Posteo password.

10. Click on Add Account.

If you have more than one calendar in your Posteo account, you can now decide which of the calendars to synchronize with Vivaldi.

11. Once you've selected the calendar(s) that you'd like to synchronize, close the setup menu by clicking Save Account.

Done. You have successfully set up your calendar in Vivaldi.