Carols began as an old English custom called wassailing, in which villagers toasted neighbours to a long life. They would go from door to door, singing for coins with which to buy cider (and ale) to celebrate the Solstice.
Many carols which have gained popularity today were printed in Piae Cantiones, a collection of late medieval Latin songs which was first published in 1582. Early, Latin forms of carols such as “Christ was born on Christmas Day“, “Good Christian Men, Rejoice” and “Good King Wenceslas” can be found in this book.“Adeste Fideles” (“O Come all ye faithful”) appears in its current form in the mid-18th century, although the words may have originated in the 13th century. The origin of the tune is disputed.