Medieval Christmas music followed the 7C Gregorian or "plain song" tradition.
The earliest extant written English Christmas carol, `A child is boren amonges man' is found in a set of sermon notes written by a Franciscan friar before 1350.
Hubert Eyck (c 1370-1426) and Jan van Eyck (or Johannes de Eyck) (Flemish artist, c 1395–c 1441) Detail The Ghent Altarpiece
During the 14C Franciscan Richard Ledrede from Canterbury collected songs, while he was Bishop of Ossory. However, he limited the collection for use by vicars, priests & clerks during the Christmas season - not the laity. He wrote that the religious may sing these songs "in order that their throats & mouths, consecrated to God, may not be polluted by songs which are lewd, secular, & associated with revelry."
Jan van Eyck (or Johannes de Eyck) (Flemish artist, c 1395–c 1441) Detail from The Fountain of Life
Most Christmas carols were handed down by word of mouth over hundreds of years. Though most carols related to the Virgin Mary, the shepherds, the Magi, & Jesus’ birth, there were also secular favorites. Oral tradition songs about nature & pagan customs, such as riotous drinking celebrations refected the culture of the day. Some of these popular songs probably had their roots in Winter Solstice festivals celebrating midwinter & the rebirth of the sun, some with references to holly & ivy.
Mariotto de Nardo (1394-1424) Virgin & Child, Detail Angel musicians, ca 1400
Carol singing was not originally limited to Christmas. New Year, Easter, saints' days, & planting & harvesting times generated their own carols. Some carols were general & could be sung year-round. Only in the 19C did carol-singing become almost exclusively associated with Christmas.
Nicolo di Pietro (14-15th century) Madonna Enthroned, 1394, Gallerie dell'Accademia, Venice
The church wasn't always thrilled about joyous carol singing. The carol was prohibited as early as the mid 7C in a decree issued by the Council of Chalonsur-Saone.
Nicolo di Pietro (14-15th century) Madonna with child, saints and angels, 1390-1400, Lindenau
Church uneasiness with the laity singing carols continued for centuries. The 1209 Council of Avignon issued a similar ban on carols.
Pere Serra (Gothic-Italian style painter, active in Catalonia in 1357-1406) Madonna with Angels Playing Music, c 1390
Borrowing from ancient "pagan" secular sources in order to "intoxicate the ear" was deplored in the 14C by Pope John XXII. The 1435 Council of Basle continued issuing bans against carols.
The 1545-63 Council of Trent attempted to diminish secular tendencies in Roman Catholic church music & to encourage attention to liturgical suitability & to the clear projection of approved lyrics & words.
Simone Martini (c 1280-1344) St. Martin is Dubbed a Knight, 1317-19, Chapel of St. Martin
By the end of the Middle Ages, all the major Christian feast days had acquired picturesque customs, processions, & folk music. The folk music had its roots in popular dances & was characterized by a spirit of gaiety & simplicity. This emergence of the folk music of the people could no longer be suppressed by ecclesiastical authority.
Unknown Artist (14th century) Musical Angels, Sacro Speco, Subiaco
During the 15C, the carol continued as a popular religious song, but it also developed as art music & as a literary form. By the end of the 15C, carols appeared in a court songbook, The Fayrfax Manuscript.
Vitale da Bologna (1309-1360) Coronation of the Virgin
This dear woodcut tells a different story about religion & music. Here gentlemen dance about in a circle; while a priest, see the rosary, and a lady are apparently so dizzy that they have fallen to the floor. During the 14C Franciscan Richard Ledrede from Canterbury collected songs, while he was Bishop of Ossory. However, he limited the collection for use by vicars, priests & clerks during the Christmas season - not the laity. He wrote that the religious may sing these songs "in order that their throats & mouths, consecrated to God, may not be polluted by songs which are lewd, secular, & associated with revelry."