The "special Pokémon" idea wouldn't be 'fair'. Especially since the region (Mirius, if memory serves) supposedly has all 493 Pokémon (excluding the Generation V ones), it wouldn't be 'fair' in a way to have some Pokémon only appear on certain events.
For instance... only having a Delibird appear on Christmas would be sort of stupid.

And raising the point that has already been made: the PU team is fairly small, so making a big thing like holiday events would be difficult.

But of course, there's nothing stopping special items being on sale for a short period of time... in fact, I would love this to be the case.

Welcome to Pokémon Universe, buddy.

Enjoy your stay, we're all pretty friendly.

Oh Gosh. I've started a racial thing.
Yeah, I'm atheist, but I respect everyone's religious beliefs and I'm sorry if I've offended anyone.

My real Pokémon would have to be a Weavile evolution that looks a little like Roserade, with the 'mask' and 'hair'.

Thanks for such a warm welcome. Going to like it on these here forums, me thinks.
I'm really looking forward to this game.