Deck the Halls

"Deck the Halls" (original English title: "Deck the Hall") is a traditional Yuletide and New Years' carol. The "fa-la-la" refrains were probably originally played on the harp. The tune is Welsh dating back to the sixteenth century, and belongs to a winter carol, Nos Galan. In the eighteenth century Mozart used the tune to "Deck the Halls" for a violin and piano duet. The repeated "fa la la" is from medieval ballads and used in Nos Galan, the remaining lyrics are American in origin dating from the nineteenth century.

Source: Wikipedia