In the holiday part, the kids are already grown ups, and there is no reason for the parents to stay together anymore, and the time for them to separate that's why it is a holiday for them.
My mom and dad were never married because my dad has another woman, and it hurts,. Why just can't they marry for me?, but I realize that if they are married, still it wont work out, and I think it's better if they are not married than being together fighting, staying together for me but there is no love at all.