XFCE has a calendar, called Orage, but it doesn't sync with Google Calendar. For those of us with an Android mobile who like to get notifications of events from our PC calendar, there is an alternative in the Debian repositories.
Iceowl is a calendar extension for Icedove (Versions of Mozilla's Thunderbird and Lightning extension) and there's a Google Calendar add-on for Iceowl.
With these packages installed, open Icedove and check Calendar in Events and Tasks.
Navigate to File > New > Calendar and check On the Network.
Open Google Calendar in a browser and click the drop-down menu next to your calendar.
Select Calendar settings, then click on the XML button next to Calendar Address:
This will give you an address to copy and paste into the Location box in the On the Network dialogue box above.
Enter an event in your calendar, click on Synchronize,and it should appear in Google Calendar.