Personally speaking, I have found the desire to share with everyone has grown as I am accepting myself more, and I guess I want that elsewhere, even though I've not yet begun any transition. I haven't even seen a therapist yet, but I figure I will before I begin sharing with everyone.
I also would NOT drop the bombshell at Christmastime, as emotions tend to be heightened around this season. You know your family though, but for me, I'd want to do it any other time than the holidays. In my case I wouldn't want negative reactions to overshadow the holidays. I'd rather deal with that any other time of the year.