Here is an interesting excerpt from David Vernier, “From Christemasse to Carole,” in the current issue of Listen magazine (article is pages 39-42).
He notes that we don't know what part of the year Jesus was born, and that the December 25th date of Christmas was probably chosen because it was already a non-Christian holiday, the solar feast Natalis Invicti on the Roman calendar's winter equinox. “However it happened, once the time of year was official determined (probably sometime in the fourth century), the course of Christmas music history was set. Not only did the ‘bleak mid-winter’ become one of the more vivid and affecting images of the season, but a whole body of songs, hymns and carols began to capitalize on the dramatic possibilities of cold, snow and wintertime activities and necessities. The shepherds in the fields, the journey of Mary and Joseph, the stark rudeness of the stable, the baby wrapped in swaddling clothes in the manger, the brilliance of the stars--all took on a more compelling aspect in the context of a cold and dark winter.
"Carols were especially good at conveying these many moods--elation, wonder, appreciation, reverence--and their texts, written in the local vernacular, told compelling stories. The carol, from the French carole, was originally a type of dance performed in a circle. The music was characterized by a refrain sung before and after each verse---and often there were many, many verses. Carols were composed and sung for all sorts of occasions and were not specifically tied to Christmas. Today the term is almost exclusively applied to Christmas music--and many of the pieces we call carols are technically hymns or songs” (pp. 41-42).