This has to be my favourite episode of the Justice League series. The episode, “Comfort and Joy” was from the second season and was written by Paul Dini (of course). This will definitely be included in the annual Christmas shows rotation.
I love how Clark still believes in Santa.
Synopsis from Wikipedia:
After saving two worlds, the members of the Justice League decide to take a break to celebrate the holidays. Flash spends his Christmas Eve with the children of an orphanage, who ask that he find them a special animated toy duck. The popular toy is broken by the Ultra-Humanite, who eventually comes around and helps Flash. Green Lantern and Hawkgirl show each other how they spend the holidays. Lantern engages in a snowball fight with Hawkgirl. Hawkgirl reciprocates by starting an interplanetary barroom brawl. Batman spends Christmas on Watchtower duty; Wonder Woman is not mentioned at all. Meanwhile, Superman invites J’onn over to Smallville for Christmas Eve with his parents. Although J’onn is initially uncomfortable, he eventually learns to enjoy Christmas. This was the first standalone episode of the series.