Christmas Mode is a secret special mode on Tyrian which is automatically activated if the computer's date is set to December.
Upon starting the game, a prompt will show asking if you want to activate the mode (you have the option to disable it if you so wish). When active, all of player's ships and weapons graphics are modified with a Christmas motif and all the narration voice is replaced as well. The changes are purely cosmetic though, not affecting the gameplay.
You can activate Christmas Mode any time of the year by typing in "FILE0001 yesxmas" when launching the program from DOSBox.