Yule is a celebration of the Winter Solstice, observed by Pagans and Wiccans.
The first day of winter and shortest day of the year represents a celebration focusing on rebirth, renewal, and new beginnings as the sun makes its way back to the Earth. A solstice is an astronomical event that happens twice each year when the sun reaches its highest position in the sky.
In the Northern hemisphere, the winter solstice has been celebrated for millennia. The Norse peoples, who called it Jul, viewed it as a time for much feasting and merrymaking. In addition, if the Icelandic sagas are to be believed, this was a time of sacrifice as well. Traditional customs such as the Yule log, the decorated tree, and wassailing can all be traced back to Norse origins.
Four thousand years ago, the Ancient Egyptians took the time to celebrate the daily rebirth of Ra, the god of the Sun. As their culture flourished and spread throughout Mesopotamia, they began to realize that every year they witnessed a cycle of birth, death, and rebirth, in relation to the Sun.
Winter festivals were also common in Greece and Rome, as well as in the British Isles. In Rome, Saturnalia was a week-long party held in honor of the god Saturn and involved sacrifices, gift-giving, special privileges for slaves, and a lot of feasting. Although this holiday was partly about giving presents, more importantly, it was to honor an agricultural god.
source: learnreligions.com
Later departing from its pagan roots, Yule underwent Christianised reformulation, resulting in the term Christmastide. Some present-day Christmas customs and traditions such as the Yule log, Yule goat, Yule boar, Yule singing, and others may have connections to older pagan Yule traditions. Cognates to Yule are still used in the Scandinavian languages as well as in Finnish and Estonian to describe Christmas and other festivals occurring during the winter holiday season.
The Wild Hunt
Scholars have connected the month event and Yule period to the Wild Hunt (a folklore motif depicting a ghostly procession in the winter sky), the god Odin (who is attested in Germanic areas as leading the Wild Hunt and bears the name Jólnir), and increased supernatural activity, such as the Wild Hunt and the increased activities of draugar (a Scandinavian folklore dead creature) who walk the earth.
Mōdraniht, an event focused on collective female beings attested by Bede as having occurred among the pagan Anglo-Saxons on what is now Christmas Eve, has been seen as further evidence of a fertility event during the Yule period.
The events of Yule are generally held to have centered on midwinter (although specific dating is a matter of debate), with feasting, drinking, and sacrifice (blót).
The Yule log, Yule clog, or Christmas block is a specially selected log burnt on a hearth as a winter tradition in regions of Europe, particularly the United Kingdom, and subsequently North America. The origin of the folk custom is unclear. Like other traditions associated with Yule (such as the Yule boar), the custom may ultimately derive from Germanic paganism.
The custom of burning a Yule log for one or more nights starting on Christmas Eve, formerly widespread in France, has now long been replaced by the eating of a log-shaped cake, also named Bûche de Noël.