Carols, as songs of joy and celebration, can actually be sung at any time of the year but in our modern world, we tend to associate the term "carol" with Christmas. Interestingly enough, what we call Christmas Carols didn't start out as celebrations of Christmas at all, but as pagan celebration songs for the Winter Solstice (the shortest day of the year). When early Christians celebrated Christ's birth at the same time, they appropriated the custom of singing the celebration songs and substituted Christian words, sentiments, and prayers for the pagan ones.
One of the earliest Christmas carols is a song known as Angel's Hymn, believed to date from A.D. 129. Other Christmas hymns appear in the eighth century as part of the Byzantine celebration that begins on Christmas Eve with what's called the Service of the Great Hours. But what we think of as Christmas carols can probably be best attributed to St. Francis of Assisi, who put together Nativity Plays in 1223 and requested that songs, or "canticles," be sung to accompany them. These "carols" then spread through Europe and became the basis of many songs we love today.
Other early carols were only loosely based on the Nativity story and contained such embellishments and "poetic license" that they were much more often sung in homes rather than in churches. Even home singing of carols fell by the wayside for a while, when the Puritans banned Christmas celebrations of any kind. Fortunately, the songs stayed alive, and people passed them along until eventually, in Victorian times, the custom of lavish Christmas celebrations and Christmas carol singing returned.
Now, carols so are much a part of our Christmas holidays that - despite the "overkill" we get of Christmas music at the mall! -most of us have favorite Christmas carols that we love to hear and even sing ourselves. Christmas caroling is alive and well today, too - from groups singing at hospitals, nursing homes, or other public venues to the "flash mob" performance of the Hallelujah Chorus at a major department store. If you get a chance to play or sing Christmas carols, take the opportunity; it's one of the best ways to really get the "Christmas spirit."