The winter solstice is an annual occurrence that marks the shortest day of the year. During this time, the sun appears lowest in the sky and the Earth receives less sunlight- the days are shorter, and the nights are longer. This is due to the rotation of the Earth around the sun. In December, the northern hemisphere is tilted away from the sun, and the winter solstice marks the time of the year when the northern hemisphere of the Earth is tilted farthest away from the sun. The opposite is occurring in the southern hemisphere. There the winter solstice is celebrated in June because in December, the southern hemisphere is tilted closer to the sun and in June, it is tilted away from the sun due to the rotation of Earth on its axis.
If you look back in history, the winter solstice historically has been a time of celebration for many different religions and cultures to celebrate the winter season and the return of light after harvest. There has been proof of humans observing the winter solstice as early as 10,200 BC (around the end of the Stone Age). These traditions and celebrations often are accompanied by fire and light to celebrate the return of light. This is because after the winter solstice, days start becoming longer and the nights shorter as spring approaches.