|The following is based on Fallout Shelter and some details might contradict canon.|
Overworked? Underappreciated? Funny uniform? Vault Dweller or Christmas Elf - the choice is yours.”— In-game description
The elf outfit is green and red, with a green hat and green boots with the toes curled up. It provides +3 intelligence and +2 luck to the wearer.
If successful attempt disassembly of outfits you can get junk:
Elf outfit (1)
Free space in the storage room
Toy car (4)
Teddy bear (2)
The outfit was added with the 1.2 update, two weeks before the start of the winter 2015 Holiday season, with events like Halloween and Christmas coming up.