Answering a month late, my bad.

Avian, Arryn and Stahl always get together for time together during the winter holidays no matter what even though they live in different regions (and Stahl and Arryn are legally separated but on good terms now. Loredrop!)

Jasmine has the classic Family Dinner at her parents house but also all of her pokemon team has a Secret Santa activite between eachother

MERRY CHRISTMAS

And for Tyler, he normally settles down in his family home back in Blackthorn City. Mainly because he's constantly traveling and loves the warm family environment. One tradition his family does every year is help set up those giant city Christmas trees over in town.

The Gym Leaders Clair, and the other civilians help out as well and even some of the town's wild Pokemon! Afterwards, Tyler puts the star on the tree since he has Dragonite to help with that, and everyone stands around and enjoys a nice Christmas fest before events like Secret Santa happens