You have procrastinated and procrastinated, and finally, you decide you just can't take it anymore and you want to end the marriage. You can't imagine going through the ordeal of Christmas faking it - so you tell your spouse. I call that the "Christmas Bomb."
When you're married in a nuclear family, the circle is around all of you -- and that's it. Everyone else is outside of it. No gaps. No blurriness. No ifs or maybes. But when that circle breaks up, you can't entirely make a solid, new one in a stepfamily.