From Bulbapedia, the community-driven Pokémon encyclopedia.
Christmas is a winter holiday celebrated throughout the Pokémon world. Much like the secular aspects of the Western holiday in the real world, people wait for Santa Claus to come down their chimneys and deliver gifts and goodwill to them. It is unknown if the religious aspect of Christmas is celebrated, though most likely not.
Episodes featuring Christmas
Manga chapters featuring Christmas
Magical Pokémon Journey
Pokémon Get da ze!
Pokémon Pocket Monsters
Pocket Monsters DP
Pocket Monsters BW
Books featuring Christmas
Cards illustrated featuring Christmas
Other media featuring Christmas