This has already been discussed in the past and rejected. That's what 
the side-by-side view is for.

If you're viewing multiple calendars, the only way to know which event 
belongs to a certain calendar is to go and enable one calendar at a 
time until you find the right one.  This is cumbersome when comparing 
multiple shared calendars, where events are owned by the same person.   
And even then, it's not easy to tell which event a calendar goes with 
just by the owner, since multiple owners can have events in the same 
calendar.



It would be very helpful if the calendar name would be visable during 
the mouseover event.