My biggest surprise came from Monkey Island 3. I was playing it on Christmas, and at one point Guybrush asks a character how long he's been sunbathing. He answers: "Since June. By the way, merry Christmas!" I later found out the game actually checks the calendar of the computer to determine what the character says. I just though he always says "merry Cristmas" and it was just a coincidence that I was really playing it on Christmas.

Dude, Stronghold is not really a serious game. There's a lot of comedic content in there.

Now a WTF Christmas easter egg can be found in Operation Flashpoint (probably also ARMA:CWA, never checked in ARMA 1 or 2). I got the GOTY edition for Christmas a long time ago. I installed it right away and WTF, many trees were turned into christmas trees. Now THAT was fucked up. I was almost sad when the Christmas was over and they disappeared. :D

But still far more serious than aforementioned Dungeon Keeper ;)

Not in my opinion. Despite the supposedly realistic medieval theme I consider it much more lighthearted than Dungeon Keeper (at least the first one). Sure, there's humor in Dungeon Keeper but it doesn't ever get silly as Stronghold - especially the characters are pure comedic relief.

Fire up Tyrian 2000 to "Activate Christmas".

I actually just booted up the game myself just to see for myself, and, yes, there it was. Now I'll have to check again on Christmas Day.