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

: Introduced alongside the holiday content, this mode allows you to restart the campaign with all your weapons, skills, and gadgets while introducing tougher enemy spawns and weapon upgrades. Related Gameplay Improvements (v1.0.4)

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....

The Merry Christmas Update 1.06 introduced several holiday-themed and gameplay enhancements: : Introduced alongside the holiday content, this mode