Though this doesn't really have much to do with the holidays, my biggest pet peeve is people pronouncing commonly used words incorrectly, though I usually don't say so as it makes me sound "snobby."
For instance...
It's REALTOR, not REALITOR.
It's ESPECIALLY, not EXPECIALLY. And along those same lines, it's ESCAPE, not EXCAPE.
It's JEWELRY, not JEWLERY.
And one of the ones that's like nails on a chalkboard: It's HEIGHT, not HEIGHTH. There's no TH sound at the end. Ugh! With so many people saying HEIGHTH, it's strange that you never hear people say WEIGHTH. :p