My Collins dictionary defines a carol as a joyful hymn or religious song, especially one celebrating the birth of Christ.
The trouble with that definition is that some carols are really rather gloomy:
and some are not religious at all (Deck The Halls); some commemorate the death of Christ (My Dancing Day); and some celebrate eating (We Wish You A Merry Christmas) and drinking (various Wassail Songs).
Luckily this doesn't matter. To carol is to sing in a joyful and enthusiastic manner, and whatever we're singing about it'll lift our hearts.
As for the others within earshot, the best thing for them will probably be to join in lustily and try to drown us out.
Thing To Do Today: carol. If you don't enjoy singing then a carol is also an old English circular dance - by which I mean one danced by a circle of people, not by a lonely, waveringly rotating drunk. The word carol came from France in the 1200s, but where it came from before that is a mystery.