In antiquity, midsummer fires were lit in high places all over the countryside, and in some areas of Scotland midsummer fires were still being lit well into the 18th century. After Christianity became adopted in Britain, the festival became known as St John’s day (24 June) and was still celebrated as an important day in the church calendar; the birthday of St John the Baptist. Traditionally St John’s Eve (23 June) (like the eve of many festivals) was seen as a time when the veil between this world and the next was thin, and when powerful forces were abroad. Vigils were often held during the night and it was said that if you spent a night at a sacred site during Midsummer Eve, you would gain the powers of a bard, on the down side you could also end up utterly mad, dead, or be spirited away by the fairies. Mysterious Britain.

So you have a choice of occasions on which to celebrate;
Summer Solstice 21 June, the longest day
St John's Eve, 23 June
St John's Day, 24 June

The Fête de la Saint-Jean (St. John’s Eve) is celebrated in France at sunset on June 23rd. St. John’s Day is actually on June 24th and it celebrates the birth of St. John the Baptist (6 months before Jesus’s birth). The festival has its roots in the ancient celebration of the summer solstice. The pagans would light bonfires on the eve of the solstice. Originally, they were lit to “help” the sun whose course would shorten until the winter solstice. During the Middle Ages, people thought the bonfires would keep sorcerers from passing by on that night.

It’s in part a celebration of the young ladies and men. On the 23rd, they go around collecting branches for a bonfire to be lit at night. Sometimes when the fire starts to die down and is very low, young people jump over it to show their virility. There’s often a nice meal involved in the festivities. Sometimes fireworks are lit and there’s singing and dancing. On St. John’s Day, French people pick St. John’s Wort. It’s put in a bottle with oil (usually olive) to make “red oil” which is used to relieve rheumatism. mamalisa.com