'Tis the Season

Seems to be a lot of confusion about the hats this year (post Cataclysm 2010).
Here is my experience, disregard the "dates" in the Blizzard statement everyone is throwing around, as it seems to be inaccurate.

Northrend Dungeons:
Grand Magus Telestra in The Nexus: Red Winter Hat
Heroic: 100% DROP!
Normal: 0% - 3% Approximate DROP!
Jedoga Shadowseeker in Ahn'Kahet: The Old Kingdom: Red Winter Hat
Heroic: 100% DROP!
Normal: 0% - 3% Approximate DROP!
Mage-Lord Urom in The Oculus: Green Winter Hat
Heroic: 100% DROP!
Normal: 0% - 3% Approximate DROP!

Cataclysm Dungeons:
Lord Godfrey in Shadowfang Keep: Green Winter Hat
Heroic: 100% DROP!
Normal: 3% - 14% Approximate DROP!
Drahga Shadowburner in Grim Batol: Green Winter Hat
Heroic: 100% DROP!
Normal: 3% - 14% Approximate DROP!
High Priestess Azil in The Stonecore: Red Winter Hat
Heroic: 100% DROP!
Normal: 3% - 14% Approximate DROP!
Corla, Herald of Twilight in Blackrock Caverns: Red Winter Hat
Heroic: 100% DROP!
Normal: 3% - 14% Approximate DROP!
Admiral Ripsnarl in The Deadmines: Red Winter Hat
Heroic: 100% DROP!
Normal: 3% - 14% Approximate DROP!

As always your mileage may vary as far as the NORMAL MODE drops are concerned, all in all if your REALLY want the hat, run a heroic.
There is of course always the slight less than 1% chance you can loot either of these hats off of any HUMANOID mob in the game.
Don't count on it however.

Please be mindful that Wowhead receives it's data from it's users, so the loot tables of the for-mentioned Bosses can be, and probably are inaccurate to some degree just from not many people looting the hats and uploading the data.
Also, the bosses get looted thousands of times when the holiday even is NOT going on, so if the hat appears on the loot table, the drop percentage is likely horribly inaccurate as well.

Happy Holidays!

Quick guide on how to complete this achievement in case anybody is having difficulty:

First, you must be wearing the following items, which are part of the item set [itemset=761]:

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  [item=21524] or [item=21525]
  
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  • Found from various instance bosses (more likely) and some world drops. Binds on pickup!
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  Red Winter Clothes or Green Winter Clothes
  
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  • Crafted by a tailor of at least level 250. Does not bind to character!
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• Winter Boots
  
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  • Crafted by a leatherworker of at least level 285. Does not bind to character!
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To complete the achievement, simply eat a Graccu's Mince Meat Fruitcake. These can be obtained through a Winter Veil present. Make sure the durability of the Winter Garb clothing is not 0 or you might not get credit!

Any 3 pieces will grant the set bonus so you should get credit regardless of the colours.