From Stonehenge in England to the Kokino observatory in Macedonia, people gather every year to mark the winter solstice.

Dec. 21 is the shortest day of the year, in terms of daylight hours, in the Northern Hemisphere.

At 12:11 p.m. ET on Dec. 21, the hemisphere begins its tilt back toward the sun, marking the winter solstice in this part of the world and slowly leading to longer days.

The solstice is the result of a tilt in the Earth's axis as it orbits around the sun, which affects the number of daylight hours. Although the arrival of the solstice cannot be seen, per se, the moment describes the instant when the Earth's axial tilt is farthest away from the sun, resulting in the shortest day of the year as well as the longest night of the year.

The Northern Hemisphere leans farthest away from the sun at this time of year, making the solstice — derived from the Latin words sol (sun) and sistere (stand still) — the shortest day of the year and the first official day of winter.

While some may not think it a cause for celebration, the solstice is a day of deep historical and cultural significance.

Celebrations through history

In earlier civilizations, the winter solstice was among the biggest festive events of the year.

Scandinavia's Norsemen called the festivities Yule. The old Norse traditions are now mostly lost to history, but some — such as yule logs, mistletoe, ham for dinner and even the word "yule" itself — have become hallmarks of contemporary Christmas celebrations.

The Roman Empire's solstice festival of Saturnalia was one of the most significant events in its calendar. Over time, the celebration grew from a one-day feast to a full week of merrymaking.

There were public holidays, with animal sacrifices and feasts. It was the only time of the year in which gambling was officially allowed, even for slaves. Slaves also enjoyed a temporary upending of social roles: they were exempt from punishment and celebrated banquets at which they were often waited on by their masters.

But other festivities would eventually eclipse Saturnalia. Christianity took hold in the Roman Empire, and Christmas became a more prominent event.

The Jewish festival of Hanukkah, the African-American celebration of Kwanzaa, the Greek festival of Kronos and other celebrations also occur at this time of year.

Shedding light

Nowadays, the solstice itself passes mostly unobserved, although there are still some groups that formally celebrate the event.

For Wiccans, Yule is one of eight solar holidays, marking the rebirth of the Great God. Druids gather at Stonehenge in Wiltshire, England, to end their mourning for "the death of the light."

I like these cold, grey winter days. Days like these let you savour a bad mood. - Calvin and Hobbes

People also gather at the Kokino mega-observatory near the town of Kumanovo in Macedonia. Dating back to the Bronze Age, this ancient observatory contains unique stone markers that track the movement of the sun and moon on the eastern horizon.

The solstice comes at a time of dark days and sometimes darker moods. After the solstice, each day gets progressively longer, but the lack of light at this time of year is known to cause seasonal affective disorder (SAD), a mood disorder affecting an estimated half a million people worldwide every year.

The acronym might be SAD, but the disorder is not just a simple case of wintertime blues. Symptoms include overeating, anxiety, lethargy, depression and loss of libido.

It's easy to fix, though: in 85 per cent of cases, exposure to an extra one to two hours of bright light a day causes notable improvement.