It is the Winter Solstice. This is the shortest day of the year. From today you will observe shorter nights and longer days. AKA the hibernal solstice it happens with either of Earth's poles reaches a maximum tilt away from the Sun.
It happens for each hemisphere and indicates that the Sun is at its lowest daily maximum elevation in the sky.
Along with the winter solstice commences Yule. It began as a part of those festivities held on the Winter solstice. It features candles and lights which symbolize guiding becon for Yeshua the Christ. Some feel that these evolved from the Yule Log which was lit to entice the sun to return to the Yule festival in scandinavia, the origin of the festival.
There are a number of pagan beliefs from which Yule evolved. This include the Holly King handing over power to the Oak King. This ushers in a new season of light according to tradition.