First Day of Winter (or Summer): December 21

Well, today is the first day of winter in the northern hemisphere. The days have been getting shorter, and now we've finally reached the winter solstice, the shortest day of the year. The solstice marks the moment the sun shines at its most southern point, directly over the Tropic of Capricorn.

What happens in the winter solstice? The winter solstice is an astronomical event, based on the Earth's tilt away from the sun. On this day, the tilt of the North Pole is positioned the farthest away from the sun, causing less light to reach the northern hemisphere. 

Elsewhere, people in the southern hemisphere will experience the longest day and shortest night of the year, which is the exact opposite of what's happening in the north.

One tradition in the winter solstice is having revellers gather at Stonehenge to watch the sunrise on the shortest day every year. This is a major pagan festival and has been celebrated for thousands of years.

So wherever you are, enjoy your first day on winter or summer.