The custom of caroling and Christmas carols is thought to have originated in England when roaming musicians would go from town to town and visit castles and other houses of the rich to give unrehearsed performances. But there also is the notion that singing carols at Christmas probably came from the group of angels, shepherds and who visited Jesus at his birth, since they worshiped the holy child, sang and proclaim praises unto Him. And after their visit, they went on with their declarations in the street.
Read more: The History of Christmas Carols