Serious.sam.siberian.mayhem.merry.christmas.v1....

: A unique cosmetic for the XPMR "Burner" Raygun, unlockable by finding a Christmas Tree and completing a special portal challenge on each of the game's five levels.

If you were specifically looking for features from the update (which arrived earlier in August 2022), it added: Guide :: Christmas event: XPMR "Freezer" Raygun weapon skin Serious.Sam.Siberian.Mayhem.Merry.Christmas.v1....

: By spending a S.A.M. skill point on the "Christmas Time" perk at the end of the New Game+ upgrade list, players can permanently enable holiday content. This allows you to visit the unique Christmas areas and earn the exclusive weapon skin at any time of the year. : A unique cosmetic for the XPMR "Burner"