A: We would rule that any legitimate holiday you can think of should work. No making stuff up…”Yeah, it’s National Elevator Day! That’s the ticket!…” unless it really is National Elevator Day. You don’t have to specifically celebrate that holiday yourself.
It may be a good idea to apply a house rule about “no looking up today’s date during the game to see if what holiday it might be.” I think in our house, we’d probably rule that you could use the internet to verify, but not search for a holiday mid-game. But that would be up to your gaming group.