When your kids are teenagers it's hard to get them very interested in Christmas. They refuse to go to carol services, lie in bed until midday on Christmas morning, nursing the hangovers they've acquired the night before, announce they're starving an hour before the meal is ready and start making appetite-destroying bacon sandwiches, then sit there texting their mates to work out the earliest they can get away from the obligations to hearth and home and reunite in a ravening wolf pack. They appear to have no interest in the simple joys of togetherness and give every appearance of preferring to be with their peers.
But then they leave home and go to "uni" and work, go and live in flats and houses alongside people who often are even less scrupulous about washing up than they are, run out of money in October, endure their first bout of flu away from the consoling arms of mother and generally come face to face with the truth that they're not all that special.
At that point they start becoming very concerned that Christmas is going to be observed according to the rules. They ring home to make sure you've got a tree, find out when it's being decorated, check that nothing unusual is being ordered in the way of food and do everything in their power to make sure that everything is being done "the way we've always done". It's interesting. You only find out about "family tradition" when you depart from it unknowingly and it's generally somebody in their twenties who reminds you of it.