Carols were first sung in Europe but were traditionally sung by pagans on the winter solstice. The word carol translates to dance or a song of praise and joy. Pagans would sing about the seasons and dance around a circle of stones. Soon after, Christians took over and gave Christian songs to people to sing.
During the year AD 129, a Roman Bishop declared that a song titled, " Angels Hymn" should be sung at a Christmas service in Rome. Soon after this, Christian Christmas carols began to become popular with all the European composers writing them. However the fad ended soon after because all the carols were being written in latin; a language not many people new or spoke. Sadly, by the middle ages, people gave up on celebrating Christmas as a whole.
Then, in 1223, St. Frances of Assisi started Nativity plays in Italy. Some were in latin but others were in more popular languages. The new carols soon spread to France, Spain, Germany, and other European countries.
The earliest Christmas carol was first written in 1410. There are only remnants left today but it is known that it was about Mary and Jesus meeting different people in Bethlehem.
My source that I used was:
"The History of Christmas Carols on Whychristmas?com." The History of Christmas Carols. James Cooper. Web. 1 Dec. 2015.