The Armour-Bewitching Charm caused a suit of armour at Hogwarts to sing Christmas carols.
References from the canon
This charm was used as part of the Christmas decorations in 1994. Unfortunately, a suit of armour so enchanted is still not a particularly clever thing, so many of them didn't know all the words to the songs. Peeves took to hiding inside the armour and filling in the gaps with lyrics of his own invention, usually very rude ones (GF22).
From the Web
"Suit of Armour" on Harry Potter Wiki