In the middle Ages, Christmas celebrations were quite different. People used to spend Advent time in peace and silence, praying and keeping a pious behaviour until 24th December, when they all would go to to sleep to wait for Christmas.
On December 25th we celebrate Christmas and Jesus birth. The following days we do not celebrate, but in the past Christians used to celebrate every day after Christmas until January 6th ( The Epiphany). This carol, is a beloved one, dedicated to the 12 Days of Christmas.