Max takes his turn in the Santa suit.

Get Smart never had an official Christmas episode but the events of the Bowers Department Store case evidently take place at Christmastime because Agent 12 is seen undercover acting as the store's Santa Claus. [Episode #4: "Our Man in Toyland".]

Three years later, Maxwell Smart himself dons a Santa suit (because the CONTROL costume party is coming up and it's the only one left) as part of an effort to aid Agent 99 when he believes she is being targeted by KAOS. Unfortunately, his attempt at hiding in plain sight backfires due to the fact that, as 99 observes, "he's dressed a little strangely for August". [Episode #93: "A Tale of Two Tails".]

Evidently CONTROL also marks Christmas with a party. The event was once crashed by the KAOS master of disguise known as The Chameleon who made a total fool of himself disguised as the Chief. [Mentioned, Episode #94: "The Return of the Ancient Mariner".] And scientist Dr. Madre has been known to make extra money wrapping for Ohrbach's during the Christmas rush. [Mentioned, Episode #95: "With Love and Twitches.]