Well Christmas of course, halloween, Easter, Thanksgiving (we don't have it here in France, so I want to have it in my game at least), New Year.
Then just for fun Epiphany so my sims can have Galette Des Rois/Kings Cake (so yummy!!) and get the trinket (yes, it's a tradition in France and I agree putting a trinket inside a cake is a weird idea, which is why I always check my slice before eating it, wouldn't want to lose a tooth :D ).
Chinese New Year would be fun too with the parade, decorations, red envelopes and raviolis.