Which christmas carol can the hogwarts suits of armour be heard singing in goblet of fire?

The Christmas carol that the suits of armor were enchanted to sing was
O Come, All Ye Faithful. Unfortunately, it would seem that the spell was designed to works on humans as the suits of armor could not sing the whole song as the knew only part of the lyrics. Peeves would then hide in them and complete the songs often with rude made up lyrics.