Summer Solstice
Revellers at the summer solstice 2013 celebration at Stonehenge Reuters

Thousands of people are set to gather at Stonehenge in Wiltshire on Saturday June 21 to mark this year's summer solstice. But what -- and why -- will they be celebrating on what is forecast to be one of the sunniest days so far this year.

What is the summer solstice?

A solstice is an astronomical event that occurs twice a year when the Sun reaches its highest or lowest point in its path as seen from the Earth, as the planet orbits the Sun.

The summer solstice usually marks the midpoint of summer and is the longest day of the year with the longest period of sunlight in the year. The winter solstice, conversely, has the shortest period of sunlight in the year.

What causes the summer solstice?

Summer solstice earth spinning diagram
Illustration of the Sun's rays hitting the Earth on the day of summer solstice in the northern hemisphere Wikimedia Commons

The Earth spins on an axis, however it is not vertical, but tilted at 23.4 degrees.

On the summer solstice, the North Pole is tipped towards the Sun more than it is on any other day of the year. It reaches the maximum height it can above the horizon, and sets at the farthest point in the west.

The Earth's tilting axis also determines the seasons we experience, so Australia, closer to the South Pole, experiences its summer solstice in December.

When is the summer solstice this year?

The precise time of the summer solstice this year will occur on Saturday 21 June, at 11.51 BST in the UK (click here to see local times for the summer solstice in other countries).

However, the date of the summer solstice changes each year, and sometimes varies between 20-22 June, depending on whether it is a leap year.

This year, the summer solstice occurs on both 20 and 21 June, so you can start celebrating a little earlier.

This is because on both days, the sun will rise at 5:01 BST and set at 23:26 BST. On 21 June, the Sun will be 152 million kilometres from Earth.

Where can I go to celebrate the summer solstice?

During the summer solstice weekend, English Heritage makes parking and entry to Stonehenge free, so that sun worshippers and modern-day Druids can gather at the monument to celebrate the Alban Hefin festival, which has been celebrated for about 5,000 years.

You can also take a look at Meetup.com which has some interesting social events and solstice celebrations, such as 4am solstice meditations in Stonehenge with a yoga group from Oxford, for example.