Caroling began thousands of years ago in Europe as a pagan tradition. Carols were songs that were sung while dancing around stone circles and they were originally sung during every season. It is unclear whether. It is uncertain whether the word carole is derived from the Medieval Latin noun charaula, which is meanas a song accompanied by a flute or reed instrument, or if it is from the Old French verb, “carole” which means to sing with joy or praise.

Around the fourth century Christians adapted the pagan tradition of singing carols at winter solstice and instead began singing songs about the birth of Christ. These songs were initially liturgical hymns sung in Latin by members of the clergy. Saint Francis of Assisi is credited with being the first person to incorporate these hymns with Nativity Plays in the 1200s but It wasn’t until the early 1400s that carols began to be written in the common tongue, therefore accessible to the masses.  

During the middle ages, before caroling became a popular public activity, there were often official town mistrels called “waits”, who were sanctioned by local authorities to sing and collect money for the poor.
The act of singing while going from home to home first emerged in Victorian England when church carols began to merge with folk music. Most of the traditional carols we know qwere composed around this time.

Caroling as we know it today is a combination of caroling and an old English tradition called wassailing.  Wassailing is a traditional drinking ceremony that normally took place in the cider producing parts of England and was intedned  bring about a good harvest the following year.  Wassail is a mulled cider that would be brought from door to door and offered in exchange for small gifts.
 
There is a wassailing tradition hat has been around in Southern wales for several for several hundred years known as Mari Lwyd.  a group of men parade from door to door caroling, one man holds a horses skull on a tall stick with a cloak over it which covers the pole and the man under it. The group sings songs at the door of each house requesting admittance  and the inhabitants of the house sings back with excuses for why they can't come in. They sing until they run out of excuses at which time the caroling party is allowed inside and given food and drink. Below is a video about Mary Lwyd