Carols go back at least to Medieval Europe, and the form is associated with Christmas, though not exclusive to it.
What is the oldest known Christmas carol? What is known about it? Composer? Place of composition? Tune? Lyrics?
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Jesus Refulsit Omnium is often cited as the oldest known Christmas song in the world. Like many of the first Christmas songs, “Jesus Refulsit Omnium” is a Christian hymn. The hymn was composed in Latin by St. Hilary of Poitiers sometime in the fourth century. It is believed that he may have created the song after the first recorded Christmas celebration took place in 336 AD.
Source : Gerald Oehring & Associates
Source : Billboard
The first musical composition ever to be associated with the Christmas holiday was likely “Angels Hymn,” a piece dating back all the way to AD 129. According to The New Daily, a Roman Bishop proclaimed that people should sing the song “in the Night of the Nativity of our Lord and Saviour.” The song is so old that historians aren’t sure what it sounded like, but The New Daily reports that its singing was a solemn affair.
Source: Readers Digest
Of course there are likely to be many songs that predate christianity that were sung to celebrate Yuletide (the midwinter solstice) that have been consumed into christian mythology.
The oldest English-language Christmas carols that are still commonly sung date back to the 16th century. These include the perennially popular "God Rest You Merry, Gentlemen," and the "Coventry Carol." (The tune, Greensleeves, of "What Child is This," is of a similar vintage, but not the words.)
There are a few surviving 15th century carols that are occasionally revived, but they are not nearly as well known. They include "This Endris Night" and "The Boar's Head Carol."
"Good King Wenceslas" is about a medieval king, and has a tune dating back to the 13th century, but the Christmas lyrics weren't written until the mid 1800s.
There are hundreds and hundreds of older carols that have fallen into obscurity. Since most of them began in the folk tradition, it is impossible to know which is the oldest, and what its exact age is, and many of them likely had no one single unique composer.
It's interesting to note that there are a mix of somber religious songs with lighthearted Christmas party songs from every era!