A person doesn't have to be a Christian in order to enjoy the Christmas season and it's festivities. Much of what we call 'Christmas' is derived from ancient Pagan roots, celebrating the Earth and it's seasons and cycles. There is a difference in celebrating the RELIGIOUS aspect of Christmas and in celebrating the CULTURAL traditions of the Christmas season. Exactly the same reasons apply to the celebrations of Easter and Halloween. A person doesn't have to be 'religious' to enjoy those traditions. Birthdays are viewed by JWs in a 'religious' context, as a negative event, whereas for MOST people birthdays are simply a happy and festive CULTURAL tradition.
Excellent answer NavyTown.