The longest day of the year (also known as the Summer solstice) is on or near 21 June in the Northern Hemisphere. It is the day when the sun is at its most northern point and this is why it is "the longest day".
In the southern hemisphere, the longest day or Summer solstice is celebrated in December when the northern hemisphere celebrates the Winter solstice
On this day you can see a very old custom at Stonehenge, in Wiltshire, England.
Every June 21st hundreds of people go to Stonehenge to watch the sun rise. The sun shines on one famous stone - the Heel stone. For the Druids this is a very important moment of the year.
Stonehenge is one of Europe's biggest stone circles. A lot of stones are ten or twelve metres high. The earliest part of Stonehenge is nearly 5,000 years old. It is thought that the Druids used Stonehenge for a calendar.
The Druids were the priests in Britain 2,000 years ago. They used the sun and the stones at Stonehenge to know the start of the months and seasons. There are Druids in Britain today too
Read more about the Summer Solstice and how the sun is at different heights in the sky