The Christmas update was released on December 18, 2022 and you can either access the Christmas event by playing around Christmas time (duh) or by starting New Game + and activating the "Christmas Time" perk in the new weapon upgrade menu. Then you can unlock the "Freezer" skin for the XPMR "Burner" Raygun. You must find one Christmas tree in each of the five story levels. There are always fireworks surrounding the Christmas tree locations. Decorate the trees with the ornaments that are scattered in the vicinity. This will open a portal next to the respective tree, which leads you to a small challenge that rewards you with a weapon part upon completion. Finding all five parts will unlock the weapon skin. You don't have to finish the level after collecting a weapon part. You can quit and load the next one from the menu.

This video (not mine) shows you the unlocking cutscene, a cool reference to the SS2 medallion:
► https://www.youtube.com/watch?v=zsvBnFyfCMA

The skin can be found in the "Game Options" menu:


Thanks to ExTaZzY and MaxRadishOne for helping me find the locations.
Thanks to UrbanDeHuman for confirming that you can quit the level after collecting a present.
Thanks to PikaCommando (SeriousZone) for the video and some wheel of fortune info.

This Christmas tree can be found at the beginning of the level.
After collecting the shotgun at the end of the beach, turn left to find the tree by the water.

Facing the hill that leads you to the next area:


5 decorations:

#1-3: Two together by the rock formation, one behind the portal.

#4: Next to the tree.

#5: By the wooden structure.

Collect all the decorations and bring them to the tree to open the portal:


Defeat all the enemies in the arena to receive a present:


This contains the weapon part:

This Christmas tree is located in the "Energize" side mission at the beginning of the level. To get there, go to the other side of the crane area and turn right.

The tree is standing in front of hangar 03:


6 decorations:

#1-2: Two right by the portal.

#3-5: Two in the middle of the parking lot and one by hangar 04.

#6: On the hood of a car wreck.

Press the button to spin the wheel of fortune until it stops on the present symbol:


An exit portal will open next to the bar, but you can also keep spinning the wheel.

Other symbols and their messages and effects:

• Explosion: "Only agile ones may continue." - Fills the room with live grenades.
  
• Backpack: "Supplies for a long journey." - Spawns some health, armor, and sometimes C4.
  
• Helmet: "Mercy for a soul that wishes to be your ally. Can you trust him?" - Spawns a small Draconian Pyromaniac with "amogus drip" shoes who follows and helps you.
  
• Broken heart: "Endure the strength of an inevitable punch." - You lose half your health (no armor) if you have 20+ HP (may vary due to hidden decimals). You lose 5-10 HP if you have less than 20 and you lose 1-8 if you have less than 8. Damage is halved on Hard and Serious.
  
• Eye: "Personnel is eager to mess with you." - Spawns 5 tuxedo Reptiloids (from the "Evil Plan" secret in level 5).
  
• S.A.M.: "It will be your power." - Spawns a Sirian Artefact of Might (skill point).
  
• Sword and shield: "Brace yourself, as you are facing true warriors." - Spawns 2 Knackered Knights (from the "Bonfire Lit" secret in level 3).
  
• Heart: "Let your soul and body heal upon this blessing." - Fully heals you to 100 health and 50 armor. If you already have 90+ health, you get +30 up to 200. If you have 200+ health, you get +30 armor, even beyond the usual limits.
  
• Empty field: "Nothing happens. Is it a testament of luck or an absence of one?" - Nothing.

This Christmas tree can't be missed. Shortly after finding the quad bikes, you'll encounter the first sentry tower on a large open field.

The tree is located right next to the gate that leads you to the rest of the level:


4 decorations:

#1-2: One behind the portal and one behind the large gate.

#3-4: One on top of the wall by the house and one in the corner next to the house.

Defeat all the enemies in the arena to receive the weapon part:

This Christmas tree is located in the same area as the Burner gun.

It's right next to the weapon:


8 decorations:

#1: In front of the shed to the right of the tree.

#2: Behind the shed to the left of the tree.

#3: Behind the concrete barriers where you find the Raygun.

#4: In the corner of the garden, behind the greenhouses.

#5: Toilet.

#6: Salad.

#7-8: Two together next to the exit.

Find your way through the maze while collecting keys and killing Octanian Zealots. The weapon part is waiting at the end of the maze:

You'll find this Christmas tree after walking through the train station.

Facing the building that you come out of:


5 decorations:

#1-2: One behind the tree and one farther to the left.

#3-4: One on the concrete fence post and one behind the barrier.

#5: Behind a pillar by the trucks.

Defeat all the enemies together with your armed snowman friends to receive the weapon part in front of the large Christmas tree by the village:


Enjoy your new weapon skin:


"Freeze well!"