A Christmas carol is a carol (song or hymn) whose lyrics are on the theme of Christmas, or the winter season in general. They are traditionally sung in the period before and during Christmas. The tradition of Christmas carols goes back as far as the thirteenth century, although carols were originally communal songs sung during celebrations like harvesttide as well as Christmas. It was only later that carols began to be sung in church, and to be specifically associated with Christmas.
Traditional carols have a strong tune and consist of a verse and/or chorus for group singing. They are often based on medieval chord patterns, and it is this that gives them their uniquely characteristic musical sound. Some carols like 'Personent hodie' and 'Angels from the Realms of Glory' can be traced directly back to the Middle Ages, and are amongst the oldest musical compositions still regularly sung. Carols suffered a decline in popularity after the Reformation, but survived in rural communities until the revival of interest in Carols in the 19th century. Composers like Arthur Sullivan helped to repopularise the carol, and it is this period that gave rise to such favorites as "Good King Wenceslas" and "It Came upon a Midnight Clear."
Today carols are regularly sung at religious services, including Carol services, but advisedly not in Christingle services. Some compositions have words which are clearly not on a religious theme, but are often still referred to as carols. (See also: List of Christmas carols)
Secular songs such as "White Christmas" and "Blue Christmas" are clearly not Christmas carols, though they are also popular in the period before Christmas, and should therefore be considered to be Christmas songs.
Carols can be sung by individual singers, but are also often sung by larger groups, including professionally trained choirs. Most churches have special services at which carols are sung, generally combined with readings from scripture about the birth of Christ, often this is based on the famous Festival of Nine Lessons and Carols at Cambridge. Some of these services also include other music written for Christmas, such as Benjamin Britten's "Ceremony of Carols" (for choir and harp), or excerpts from Handel's "Messiah."
There is also a tradition of performances of serious music relating to Christmas in the period around Christmas, including Handel's "Messiah," the "Christmas Oratorio" by J. S. Bach, "Midnight Mass for Christmas" by Charpentier, and "L'Enfance du Christ" by Berlioz.
In England there is a tradition of Christmas carolling (earlier known as wassailling), in which groups of singers travel from house to house, singing carols, for which they are often rewarded with money, mince pies, or a glass of an appropriate drink. Money collected in this way is normally given to charity.
Christmas carols can also be played on musical instruments, and another tradition is for brass bands, such as the Salvation Army brass bands, to play carols before Christmas.