I'm just not sure, it seems like this would make the game unnecessarily difficult to play. It's a cool concept and would flesh out the world a bit more... I just can't see how it would be implemented in a way that wouldn't completely derail the fortress and possibly cause its destruction.
Let's break this down into two main parts.
"...make the game unnecessarily difficult to play."
Good idea, badly worded. DF is already unnecessarily difficult, it's just so in entertaining ways. Holidays? Maybe not so much.
"I just can't see how it would be implemented in a way that wouldn't completely derail the fortress and possibly cause its destruction."
Like there's ANYTHING that can't cause a fortress's destruction? And everyone seems so fond of derailing stuff here...
Well, there's "necessary difficulty" and "unnecessary difficulty". Most of the difficulty in DF comes from inexperience with the game itself. Only through losing enough times by Tantrum Spiral do you learn easy ways to prevent them. Only by losing enough times due to siege, do you learn competant and easy ways to combat those. Then you've got the flooding (magma/water) of fortresses which are engineering difficulties you learn through.
What a "Holiday" in the "unnecessary difficulty" category would do, is grind your fortress to a standstill with no way prevent or work around it. Under siege? TOO BAD! Need to trade with the Dwarves? Other problems in your Fortress rely on simply learning enough to avoid them, or enough to deal with them. A Holiday that grinds your fortress to a standstill regardless of what is going on... Would do neither. It would make the game unnecessarily difficult, and I can already see the threads that would be dedicated to its removal. Unless you somehow made them "managable" or merely "flavor text", they're quite unreasonable.
I don't mind a fortress getting destroyed because of any of the preventable reasons. I really don't. If I failed to do something properly. If I messed up something. If I got someone killed who had a lot of friends... That's fine. Even the occasional "bad luck" moment where a FB shows up and has JUST THE RIGHT composition to murder everyone. That's okay because it's just "roll of the dice" and it happens.
However, if your fortress gets obliterated because the Dwarves are too busy celebrating "Jabba-Wabba-Who-Ha Jangle" Day instead of manning their posts or building the walls... Then I have to take issue with that. If you limited the "party-goers" to simply those with the "No Job" status, then you could mitigate this a lot. However, if it's "fortress wide" and "no way to stop it", then it's going to absolutely destroy pretty much EVERY fortress you create that's not severely trap based. It might even destroy early fortresses you create before you ever get up PROPER defenses. Which would, of course, lead you to building defenses FIRST in any embark, and not even trading or doing other stuff until they were up. Which, in turn, basically wrecks the entire point of having Sieges only show up after a certain population.
Just sayin'. If you want Holidays... They at least have to be MANAGABLE. If they're not... They can be potentially game-breakingly annoying.