Hark the wotsit doo-dahs thingy

A carol was originally a round-dance-cum-song associated with the harvest festivities, but by the 13th to 14th centuries had come to mean any festive song, and quickly became connected with Christmas. Oliver Cromwell banned them in the mid-1600's, and it wasn't really until the Victorians invented our modern image of Christmas that they really took off. There are many myths connected with certain carols, one of the most enduring being that "Silent Night " was written on the spur of the moment by a parish priest on learning that the organ of his church was out of action, and needing something which could be sung to a guitar accompaniment. In fact, he'd written it several years before, and the organ was fine.