Thunderbird Supernova Preview: The New Calendar Design
This doesn't look all that different to the existing calendar really. They have moved the events section to the side instead of at the top and changed some of the sectioning going for the material view where the distinction between elements is just replaced with background coloured space. I am largely indifferent to this change, it doesn't look like its big or important or functionally impressive in any way. I am not going to collapse the weekends  and it seems to be the only other new feature being offered is messing with colours for categories. I suspect most people use calendars for categories already, this may be an improvement for some people but its really not worth trying to merge calendars for this purpose as an existing user unless they make that easy (which there is no suggestion they will).Change for fashion sake and nothing that users have been asking for, sums up all development out of Mozilla at the moment.