So you're using Thunderbird and need to sync your calendar?
Here's how you do it.
If you don't yet have it, you can download and install Thunderbird from Mozilla directly: https://www.mozilla.org/thunderbird/
Connect atmail account
Once you have installed Thunderbird, open it up and click "Create a new calendar":
You'll get a pop up window for account set up. Select "On the Network":
On the next screen, choose CalDav and enter your Calendar URL:
Note: You can find your calendar URL in the webmail by clicking on the Calendar tab, then the Calendar Options icon (cog) next to the calendar you have selected.
Choose a name for the calendar in Thunderbird, along with a colour:
All that's left after this is to enter your credentials (email and password) when prompted:
And that's it!