Your Tamagotchi has to be an adult to see the Christmas tree animation. If you don't touch your Tamagotchi for a while, it will get bored and the Christmas tree will come and dance with your Tama. It can come randomly anytime in December.
OK - i've worked out the dates and times of heaps of animations that ive posted on TamaZone, but for the Christmas tree, i'm pretty sure you do need to be an adult, it can be seen at 11.30am, 2.00pm and 4.30pm between November 30th and December 25th (but not after 11.30am on December 25th).