Kallikrates said:
But that doesn't mean we can't use our characters to eat like there's no tomorrow, can't forget the fact that everyone's going to get a lot crazier since Christmas, Kwanzaa, and Hanukkah are just around the corner.
*Puts on Waterdeep educator's hat*
Actually, if Waterdeep had a city-wide holiday, it would likely be a general'ish one. Waterdeep (like Silverymoon, to the north) has an Open Religion [law? mindset? philosophy?], which more or less means, in a nutshell, that no religion (since there are over 100 deities in FR) will be favored more (or less) than any other one.
Waterdhavians tend to be a pretty laid back and understanding of the differences, since it's 1) a mercantile city and
2) the ships that come in bring a pretty wide array genre of diversity, so there aren't too many city-wide focused holidays/festivals (note that I say city-wide ... temples, of course, still have their respective celebrations). Also because,
3) being there are a lot of people in Waterdeep who want to make a lot of money from natives (and, more importantly, tourists), it wouldn't bode well for any of the merchants to bo [overtly] discriminatory
*Takes off Waterdeep educator's hat*
The reason I suggested the Feast of the Moon festival is because it is coming up and the purpose of the holiday - to honor the dead and one's ancestors - ought to pretty much carry over to most religions - both in character and out.
Granted, some of those necrophiliac-loving folks might not honor the dead so much ... but, hey, they can still give thanks to the dead that they're dead so that they have corpses to experiment with and make undead, eh?
Though, like you say, Kal, it could easily be just a regular, old, roll-in-the-snow, make-the-best-snowman, don't-eat-the-yellow-snow Winter Ball.