There's a side of world-building that comes up whenever you have clerics - so what sorts of holidays does their religion actually have? If you go about it fairly blithely, you end up with something that looks like a mix between the medieval Catholic list of Saints' days and the solstice/equinox set-up from modern paganism. And as far as it goes, that's not bad - familiar without being too familiar. But it also has a "been there, done that" feel to it for me.
What holidays represent in game terms are a tension between referee and player creativity, or so it seems to me. Because holidays represent significant moments in time, it is very easy for referees to engage in some deep world-building regarding them - but that's not necessarily a good thing. If you build a deeply detailed religion with holidays, tenets and doctrine, you have to work to provide players with a means of adding to them or in some way creatively engaging with what you've presented. By contrast, one of the things that I actually like about OD&D's Gods, Demi-Gods and Heroes is how unfinished it is - you get stats on the deities, some information on the pantheon of which they are a member, and sometimes a little about their followers and worship. There's a lot of room for a referee and players to work in developing something for their own game. Each version of this that has followed - Deities and Demi-Gods, Legends and Lore, etc., have gotten more and more detailed about the religions. Unfortunately, that means that - like with stat blocks for monsters - it's easy to get buried under "doctrine" and "structure of belief." It's a lot of "rich background detail" - but how much do you or your players really want?
I used to want a LOT of background detail; even today, what I consider a bare-bones outline is more than some referees are probably willing to do. But now I find myself wanting to leave the background a little more mysterious - open to being left undefined or created out of game-play. So when it comes to holidays (and in-game religion), I'm come full-circle from when I was much younger - I'm willing to do some work, but I think I'd rather let a player playing a cleric help me figure out when they are. It'll be more fun that way.