Today, we celebrate the shortest daylight! (In the Northern Hemisphere).

From WikiPedia:

The winter solstice, also called the hiemal solstice, hibernal solstice, and brumal solstice, occurs when either of Earth‘s poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the Sun is at its lowest daily maximum elevation in the sky. Either pole experiences continuous darkness or twilight around its winter solstice. The opposite event is the summer solstice. Depending on the hemisphere’s winter solstice, at the Tropic of Cancer or Capricorn, the Sun reaches 90° below the observer’s horizon at solar midnight, to the nadir.

The winter solstice occurs during the hemisphere’s winter. In the Northern Hemisphere, this is the December solstice (usually December 21 or 22) and in the Southern Hemisphere, this is the June solstice (usually June 20 or 21). Although the winter solstice itself lasts only a moment, the term sometimes refers to the day on which it occurs. Other names are the “extreme of winter” (Dongzhi), or the “shortest day”. Since the 18th century, the term “midwinter” has sometimes been used synonymously with the winter solstice, although it carries other meanings as well. Traditionally, in many temperate regions, the winter solstice is seen as the middle of winter, but today in some countries and calendars, it is seen as the beginning of winter.

Since prehistory, the winter solstice has been seen as a significant time of year in many cultures, and has been marked by festivals and rituals. It marked the symbolic death and rebirth of the Sun. The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days.

Later on, that article speaks of the Celtic history:

Celtic

Prior to the arrival of Christianity, the Celtic people of Britain celebrated Yule in a similar fashion to the Germanic festival. It is alleged that Celtic Druids began the tradition of the Yule Log, with the intention of driving out darkness, evil spirits, and poor luck in the following year. The Yule Log was intended to be kept alight over the entire solstice period, twelve days over which the sun was believed to stand still. The log being extinguished symbolised poor luck in the following year. Additionally, evergreen plants were used in decoration – of key significance are “The Holly and the Ivy”, used in decoration, and Mistletoe, suspended over a doorway in a token gesture of goodwill to all who passed under it. These traditions have been adopted into the Christian winter celebrations, symbolised by a mistletoe wreath placed on the front door to a building.

It is a most ancient celebration because as soon as humans recognised that this was the shortest day they were deeply respectful of the forces of the universe.