I have been there, and I think one reason for the increasing amount of brake ups during holidays and vacations is because of expectations and the fact that we are free from work! In my experience it is easier to just live on and stay in a bad relationship, or a not so good relationship, during common and hectic days. We are so occupied with work and career, children, friends and everyday chores, that we somehow can live beside each other, not with each other, at least for a while! In some way one can say that our way of living isn't made for relationships anymore, because we are too busy and spend our time running around doing all the things that we think we must do!!
But when the bigger holidays comes, we are suddenly supposed to be happy, and to have a warm and cosy family feeling, filled with love and security. Somehow, the absence of those feelings becomes so clear and obvious during Christmas and New Years Eve.
Or maybe our expectations are unreasonable and we don't see and value the family we have.
So my answer is that the holidays make us see the problems that are already there from the beginning!