When I told my mother, I sent a very long email. I did it that way so she couldn't interrupt me while I was talking. Then I waited a few weeks before visiting and discussing it in person. I brought a book on the subject with me and asked her to read it. A couple months later I brought up the book and to my surprise, she actually read it. I don't remember the book, but I remember being in tears most of the way through reading it myself. It was like looking in a mirror. That was over 10 years ago.
I'd definitely wait till after the holidays. If they're not okay with it, their mood will seem off to everyone around them. If it goes for the worst case scenario and you're suddenly not there either, that won't help either. You want them focused on what you're saying, not on what they still need to get done to prepare.
