Between 20 June and 22 June
The longest day of the year, where the night is shortest, is known as the Summer Solstice. Differing from the cultural celebration of mid-year at Midsummer, the Summer Solstice is the single day of the year where light reaches its zenith over dark. It has been acknowledged since before the Roman invasion, noted during the Christianisation of Britain, and continues to be recognised for its astrological and meteorological value. It is sometimes known as Litha, from the Anglo-Saxon names for June and July—sē ǣrra līþa or the early Litha month and sē æfterra līþa or the later Litha month—though the use of Litha for the Summer Solstice itself is a relatively recent tradition.
Historically, the Summer and Winter Solstices were as important as the Vernal and Autumnal Equinoxes, and it is from these dates that the annual designation of Midsummer and Christmas came about. They are the heights of Summer and Winter respectively.
The ancient pagan nations observed a festival at mid-summer, or the summer-solstice, when the sun arrives at the culminating-point of his ascent on the 21st of June, or longest day.Robert Chambers, The Book of Days: A Miscellany of Popular Antiquities
Summer Solstice takes place in June, and was the signifier of the strength of the Oak King, who would rule from Spring to Autumn. The Oak King and his power would wax and wane, like the seasons or the moon, as would the Holly King who ruled the Winter. During Summer, the Oak King held sway over crops, from their first shoots to their final harvest.
The Solstice is still noted, with celebrators gathering at historical sites such as Stonehenge, where the stones are positioned to reveal sunrise on the Summer Solstice. This is marked by the Heel Stone. According to folklore, the Devil wanted to deceive future generations as to the purpose of the stones, as he had bought them from a woman in Ireland and carried them wrapped in cloth, dropping one into the river Avon and bringing the rest to Salisbury Plain. A friar denounced the Devil’s plan, so the Devil threw a stone which struck the friar on the heel, marking the sunrise.