Festive Outfits

The Festive Outfits package causes new holiday NPCs to spawn in your garrison. Guards with festive clothes will spawn, replacing about half of your regular guards. A large Snowman NPC will also spawn.

Source
This item can be bought from Izzy Hollyfizzle for.

Alliance
Jolly versions of human, night elf, pandaren and Shodo-Pan guards, and merry versions of dwarves and draenei guards. Gnomes and worgen do not get a name change. Death knights can get Lunarfall Merry Knights. Depending on the race of guard, they can wear a Red Winter Hat, Red Winter Clothes, or both. Regular dwarf female guards also have a Gaudy Winter Veil Sweater on underneath.

Glark Criswold will also spawn.

Horde
Festive versions of orc, tauren, troll, undead, blood elf, pandaren and Shodo-Pan guards, and a merry version of goblin guards. Death knights guards do not get a name change. Depending on the race of guard, they can wear a Green Winter Hat, Green Winter Clothes, or both.