Generally, the calendar is a good product. BUT: If you really use it in earnest (for example for work), then you soon notice that there are important features that are missing.
I would suggest that you add at least two very important features that even exist in terrible Outlook!
I have even switched back to Outlook because I need these two features!
a) When you add an event spanning several days, it should be shown as one single box, not as individual boxes on each day! People have suggested this since 2009, but it has never been implemented. It is a very important feature to keep your calendar tidy. Please add that...
b) In Outlook, I can use keyboard shortcuts to assing a colour to an event. In Thunderbird, you can use colours for categories (good), but it is not possible to assign a keyboard shortcut to a category (why not?). This makes it terribly complicated to assign a colour to your event: I need several mouseclicks to do so! Please add this function.