By the High Middle Ages, Christmas had become so prominent that chroniclers routinely noted where various magnates "celebrated Christmas." King Richard II of England hosted a Christmas feast in 1377 at which twenty-eight oxen and three hundred sheep were eaten. The "Yule boar" was a common feature of medieval Christmas feasts. Caroling also became popular, and was originally a group of dancers who sang. The group was composed of a lead singer and a ring of dancers that provided the chorus. Various writers of the time condemned caroling as lewd, largely due to overtones reminiscent of the traditions of Saturnalia and Yule).
"Misrule" drunkenness, promiscuity, gambling was also an important aspect of the festival.
New Year's Eve:
the final day of the Gregorian year, which was devised both because the mean year in the Julian Calendar was slightly too long, causing the vernal equinox to slowly drift backwards in the calendar year, and because the lunar calendar used to compute the date of Easter had grown conspicuously in error as well.
The Gregorian calendar system dealt with these problems by dropping a certain number of days to bring the calendar back into synchronization with the seasons, and then slightly shortening the average number of days in a calendar year, by omitting three Julian leap-days every 400 years.
See earthcam 06.