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Temperatures in the UK are forecast to reach "heatwave conditions" this week, perfectly coinciding with the official start of summer.

The exact start date of summer depends on whether you follow the astronomical calendar, or the meteorological one, according to the Met Office. Meteorological summer always starts on June 1 and ends on August 31, as the seasons are simply split into four periods of three months each. But refer to the position of the Earth's orbit in relation to the sun, taking into account and solstices. Since the seasons vary in length, it means the start date of a new season typically falls on a different day each year.

This year, astronomical summer officially starts on Saturday, June 21, and will end on Monday, September 22. The start of the sunny season is officially marked by , otherwise known as the longest day of the year, meaning it is the one day that has the most hours of sunlight.

The solstice always occurs around June 21 in the northern hemisphere when the sun travels along its northernmost path to reach its highest point of the year. It takes place when the Earth arrives at the point in its orbit where the North Pole is at its maximum tilt toward the sun and is directly above the Tropic of Cancer, resulting in the longest day and shortest night of the calendar year.

The day signals the moment the sun's path stops moving northward in the sky, with the days gradually becoming shorter and darker afterwards as we move towards winter. But the days won't become noticeably shorter for a while, with the shortest day of the year not due until 21 December, known as the winter solstice.

Historically, the summer solstice used to take place between the planting and harvesting of crops, giving people who worked on the land time to relax. This is also why many people would traditionally get married in June and why it still remains a popular month for weddings.

The longest day is a significant time of the year in many cultures and has inspired many festivals and midsummer celebrations. In ancient Germanic, Slavic and Celtic tribes, pagans celebrated midsummer with bonfires and dancing, while other traditions include having picnics, watching the sun rise and Maypole dancing.

The Pagan festival of Litha starts on the eve of the summer solstice and celebrates the power of the sun god. According to Pagan folklore, evil spirits walk the Earth more freely on the solstice, so people wear protective garlands of herbs and flowers to ward them off.

Many people also visit Stonehenge on the summer solstice to see the sun rise at the heritage site. The rising sun only reaches the middle of the stones on one day of the year, when it shines on the central altar. The sarsen stones, which were put up at the centre of the site in around 2500 BC, were carefully built to align with the sun on the solstices.

The summer solstice will be celebrated at Stonehenge this year from 7pm on Friday, June 20, to 8am on Saturday, June 21. English Heritage is also running a live stream of the summer solstice sunset on the evening of June 20 on its Facebook page and YouTube channel.

English Heritage says: "The stones near Amesbury at Stonehenge in Wiltshire, that were set up in about 2500 BC, were carefully designed to align with the movements of the sun. Standing in the centre of the monument at summer solstice, the sun rises just to the left of a large standing stone outside the stone circle, known as the Heel Stone, seen through a gap in the outer sarsen circle.

"Excavations have shown that the Heel Stone may have had a partner and so the sun would have risen framed between the two stones. The first section of the Stonehenge Avenue is also aligned on this same axis. There is a second alignment to the south-west, in the direction of the winter solstice.

"It is likely that people gathered at Stonehenge at both midsummer and midwinter solstices to conduct rituals and ceremonies relating to the changing seasons, the sun and the sky. It must have been important to align their monument with the movements of the sun but we may never know the exact reasons why.

"The longest day of the year would have perhaps been a time of celebration, with warm nights and long daylight making it the perfect time to gather together."