I still remember when The Gecko and I discovered the Christmas mode in Tyrian back in the late 90's. Neither of us had played the game in months and we were talking on the phone and one of us (I think him) decided to just load the game up out of boredom and lo and behold:
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Christmas has been detected.
Activate Christmas? (Y/N)
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Epicness! :D
Also, NiGHTS into Dreams on the Saturn has some Christmas related easter eggs if memory serves, though I've never tried to trigger them since I found out about them after the backup battery died for the first time and all my save data was lost. (I mean seriously, 2 years save battery life? My Dragon Warrior II cart for the NES, which I've had for over two DECADES, still holds its data!)
--- Kris Asick (Gemini)
--- Pixelmusement Website: www.pixelships.com
--- Ancient DOS Games Webshow: www.pixelships.com/adg