Okay, Carnival (using the one in Venice for reference) is an annual thing. The biggest part of it is the masks and costumes/disguises.
It began as a feast then evolved. Low-class, nobles, didn't really matter. Especially since they were all in costume. (As a note? Mardi Gras is essentially the same thing. They occur at the same time)
There are parades, masquerade balls, music, parties, pranks and other mischief.
Carnival is technically one day, but is generally celebrated for a week, sometimes more. Some people go out in costume/with masks the entire time.
So, we have those as options. (And I'll be adding more as I find more details, but this is a good starting point)