On Friday 21 December the northern hemisphere experiences something called the Winter solstice.
The date marks the 24-hour period with the fewest hours of daylight in the year, which is why it is known as the shortest day and longest night.
So what's the science behind it?
It happens because at this particular moment the North Pole is tilted furthest away from the sun as the Earth continues on its orbit - which is why it's winter in the Northern hemisphere at this time of the year
In the southern hemisphere it's exactly the opposite story - the South Pole is pointing towards the Sun, making it summertime 'down-under'.
In Sydney, Australia, they're having their summer solstice - mornings will start getting darker from the middle of December, and evenings will get lighter until early January.
But in the UK (and rest of the northern hemisphere) it means the sun rising earlier and setting later as we journey again towards the spring equinox.
But even though this is the point with the least hours of daylight, it's not the earliest sunset of the year, or the latest sunrise.
The mornings continue darkening until early in the new year.
The reason why is that a day - a solar day that is - is not always exactly 24 hours.
Winter Solstice has been celebrated in cultures the world over for thousands of years.
In the olden days it was known as Yule, and was a celebration of light and the rebirth of the Sun.
'Yule' is still celebrated in Germany and Scandinavia, and this is where we get many many of our Christmas customs, including Christmas trees and putting wreaths on the front door.