Did you know that the very first Christmas carols were written in Rome during the 4th century? Usually these hymns, such as Veni redemptor gentium written by Ambrose, were strictly statements of theological doctrine. These Latin chants, litanies, and hymns were intended to be used during the liturgy and were not considered 'popular songs' outside of the service.
In the 9th and 10th centuries under the leadership of Bernard of Clairvaux - the nemesis of medieval church decoration and art - rhymed stanzas were developed for music and thrived in northern Europe. And in the 13th century Christmas songs were sung in the vernacular, and what we think of as 'carols' were born. Also at this time the English added a circular dance to the singing of Christmas songs, and thus the term 'carol' was coined.
But 1426 is the magic date for Christmas carols when the poet-priest, John Audelay, set to music the "25 songs of Christmas" that were most likely sung by wassailers (remember them?!). These songs were associated with some of the earliest 'carols' that would have been sung door-to-door by the wassailers, and include such hits as "The Holly and the Ivy" and "Good King Wenceslas".
What is your favorite Christmas carol or hymn?