The origins of caroling during the Christmas holiday is uncertain and scholars tend to agree to disagree on the beginning of the tradition.
French historians write about Bishop Telesphorus of Rome whom introduced the custom of celebrating Christmas with songs about 129 A.D.
Some confusion comes because songs were often sung while pagans danced joyfully at their Winter Solstice celebrations. "Carole" is defined as Old French as "...round dance with singing." Pagans celebrated every season with their caroling - not just for winter.
Along with starting a tradition of a live nativity scene (creche) it is believed that St. Francis of Assisi brought additional joyful music (other than the traditional hymns) during the midnight mass worship services in AD 1223.
It is true that millions of people everywhere enjoy watching and listening to carolers every Christmas season. Today, many European towns and countries even house those who boast it is their hometown location for the origin of the Christmas caroling tradition. What is not disputed is that music in religious celebrations to honor the birth of Jesus was commonly practiced.