The summer solstice is the longest day of the year. In the northern hemisphere - where the UK is - the summer solstice falls on June 21 this year.
But the exact time changes each year, and every leap year it comes on June 20.
Summer solstice is celebrated by thousands of pagans across the world.
Many gather at Stonehenge which is believed to have been used as an important religious site by early Britons 4,000 years ago.
On the summer solstice, the central Altar stone at Stonehenge aligns with the Heel stone, the Slaughter stone and the rising sun to the north east.
What makes the solstice the longest day of the year?
We get the most hours of daylight on this day because of the position of the Earth in relation to the Sun. Our planet does not spin on a vertical axis - it is titled. This means the amount of sunlight that reaches different regions of the Earth changes during the year as it orbits the Sun.
Why doesn't the sun set in some parts of the world?
Around the time of the summer solstice areas of Norway, Finland, Greenland, Alaska and other polar regions experience 'midnight sun'. In the Arctic Circle the sun does not set at all. Again it comes down to the tilt of the Earth's axis.
Why is Australia's summer solstice in December?
While June 21 is the summer solstice in Britain, for Australia and countries in the southern hemisphere this date marks the winter solstice - they have their summer solstice in the middle of our winter.