Today, only the most desperate of theater kids go door-to-door to sing at people on the pretext of holiday cheer, but in the before times, it was more like trick-or-treating for Jesus. Christmas caroling likely has its origins in the ancient pagan customs of wassailing and mumming, which have slightly different traditions, but both involve knocking on doors to demand the occupants' booze.
Wassailing, named after a drink that sounds like a suspicious combination of cider and eggnog, involved going around to your wealthier neighbors' homes and either offering the offending beverage in exchange for gifts or demanding it in exchange for songs, blessings, and other empty gestures. It was originally an occasion for celebrants to dance around apple orchards, singing loudly to "scare off evil spirits" and "wake up the trees," but at some point, they decided it was more fun to hold people hostage for figgy pudding.
Meanwhile, mumming or mummering required participants to dress up in the weirdest combination of garments they could scrounge up around the house, crucially including a mask -- often a transparent veil of some kind or, in the most badass cases, a horse skull -- and invade their neighbors' homes. The occupants would then have to try to guess the identities of the "mummers" while they paraded around, singing, dancing, or reciting poetry and dramas.
Once they guessed right, everybody got drunk, and the mummers moved on to the next house. It was a controversial practice for some time, outlawed in medieval England because it had become a favorite strategy for getting into strangers' homes to rob them and in Newfoundland in 1862 after a group of mummers killed a dude. It's now the place to mum, though, having launched a whole Mummers Festival in 2009 and once again proving itself the weirdest place in Canada.
Top image: Arthur Rackham