Thunderbirds calendar is great. But its kinda random, in this mail program.
It doesnt really fit to the "one app one task" philosophy that is pretty normal everywhere. Everywhere you would have a seperate app for Mail or Calendar.
Wouldnt it be great to have a custom CSS window just launching the calendar without tabs and all that, having a different desktop entry?
The same for calendar, which would mostly make sense if good sync and desktop integration is there.