One thing I do remember asking, and no one could ever answer was if they don't celebrate the different holidays, because of pagan origin, then why couldn't they celebrate the Jewish holidays such as Hanukkah, surely they weren't started by pagans. Weren't the Jews at one time God's chosen? That was how I brought it up back then, this was before I was even baptized. No one could or would answer my question.
I'm shocked no one could give you an answer. I can make up a perfectly coherent one for you just off the top of my head - The Jews were god's chosen people, but when they killed jesus and the curtain between the holy and most holy in the temple ripped, that symbolized the breaking of the covenant that god had with them and they are no longer his people. Jesus came to fulfill the law and put an end to it, so celebrating the jewish holidays would be clinging to the tradition of people that were no longer special in gods eyes, and clinging to requirements that were ended by Jesus. This would put you in similar footing to Lot's wife who looked back at the things left behind, you wouldn't want to be like Lot's wife, would you?
That'd be a great after-the-fact doctrinal justification. Obviously, though, it's more succinctly explained as: just like the other holidays you're not allowed to celebrate, you can't celebrate the jewish ones because then you might do so with non-JWs and start to see that they aren't so bad.