Noob here, where do I begin Xmas event?

[Nobully-blackrock (Nobully)]

it changes each year.

they drop from dungeon bosses, so you need to check which bosses currently drop them

from wowhead:

Hunt a Holiday Hat

During the Feast of Winter Veil, certain humanoid bosses now sport a festive holiday hat!

The following Legion and Battle for Azeroth bosses will drop a Finely-Tailored Green Holiday Hat or Finely-Tailored Red Holiday Hat on their defeat. These are high-resolution versions of the original winter hats.

• Raal the Gluttonous in Waycrest Manor - Finely-Tailored Green Holiday Hat

• Aspix in Temple of Sethraliss - Finely-Tailored Green Holiday Hat

• Illysanna Ravencrest in Black Rook Hold - Finely-Tailored Green Holiday Hat

• King Deepbeard in Eye of Azshara - Finely-Tailored Green Holiday Hat

• Varimathras in Antorus, the Burning Throne - Finely-Tailored Green Holiday Hat

• Vol’kaal in Atal’Dazar - Finely-Tailored Red Holiday Hat

• Skycap’n Kragg in Freehold - Finely-Tailored Red Holiday Hat

• Fetid Devourer in Uldir - Finely-Tailored Red Holiday Hat

• Talixae Flamewreath in Court of Stars - Finely-Tailored Red Holiday Hat

• Krosus in the The Nighthold - Finely-Tailored Red Holiday Hat

• Saprish in the The Seat of the Triumvirate - Finely-Tailored Red Holiday Hat

The following bosses will drop a Green Winter Hat or Red Winter Hat upon their defeat:

Northrend

• Jedoga Shadowseeker in Ahn’kahet: The Old Kingdom - Red Winter Hat

• Mage-Lord Urom in The Oculus - Green Winter Hat

• Grand Magus Telestra in (Zone #4120) - Red Winter Hat

Cataclysm

• Corla, Herald of Twilight in Blackrock Caverns - Red Winter Hat

• High Priestess Azil in The Stonecore - Red Winter Hat

• Admiral Ripsnarl in The Deadmines - Red Winter Hat

• Drahga Shadowburner in Grim Batol - Green Winter Hat

• Lord Godfrey in Shadowfang Keep - Green Winter Hat

Pandaria

• General Nazgrim in Siege of Orgrimmar - Red Winter Hat

• Galakras in Siege of Orgrimmar - Green Winter Hat

Draenor

• The Butcher in Highmaul - Green Winter Hat

• Soulbinder Nyami in Auchindoun - Green Winter Hat

• Oshir in Iron Docks - Green Winter Hat

• Gug’rokk in Bloodmaul Slag Mines - Red Winter Hat

• Commander Tharbek in Upper Blackrock Spire - Red Winter Hat

• Iron Reaver in Hellfire Citadel - Red Winter Hat

• Tyrant Velhari in Hellfire Citadel - Green Winter Hat

• Beastlord Darmac in Blackrock Foundry - Green Winter Hat

• Flamebender Ka’graz in Blackrock Foundry - Red Winter Hat

Shadowlands

• Devos in Spires of Ascension - Red Winter Hat

The winter hats also have an extremely low chance to drop from any humanoid mob in the world; I wouldn’t hold your breath but who knows–you could get lucky! In previous years, heroic Northrend and normal/heroic Cataclysm dungeon bosses dropped hats. If you’re unlucky winning rolls, seems your best bet is to try to solo a Northrend heroic boss.