While the solstice date varies, Midsummer’s Eve is fixed on the calendar as 23June, pegging a moveable feast to a fixed date, and the folk customs of the solstice moved with it.
It was widely believed that spirits gathered on Midsummer Eve. In parts of England it was the convention to light large bonfires after sundown to ward off evil spirits; this was known as ‘setting the watch’. 
Midsummer Eve was believed to be one of the great fairy festivals  when fairies are abroad, moving amongst human kind, frolicking around the Midsummer bonfires and playing all sorts of tricks ranging from stealing human brides and performing innocent pranks to inflicting horrible curses and even death. In the Shetlands, the mysterious selkies come ashore. They normally look like grey seals but on this night they shed their skins to become human and dance on the shoreline. If they are disturbed they will grab their skins and run back to the sea, though if a man can steal and hide the skin he can force a selkie maid to marry him, but if she ever finds her skin she will put it on and be off back to the sea. In Russia, the green-haired Rusalka fairies walk the land at Midsummer, and where they tread, flowers appear, and when they move through the grain it causes it to grow. The mischievous Robin Goodfellow or Puck is about in English woodland, playing tricks on unwary travellers and leading them from their paths. Certainly we had a strange experience in the coven one solstice, when we turned away from the circle and couldn’t find our way back, even though it was only a few yards away and knew the woods intimately. Eventually, after walking a just few paces, we found ourselves at the other side of the woods, at least a mile away, so I do believe it happens!
According to fairy lore, if you want to see fairies then you will need the aid of certain magical herbs such as thyme. In Shakespeare’s A Midsummer Night’s Dream, Oberon tells Puck “I know a bank where the wild thyme blows/ Where oxlips and the nodding violet grows,” because at midnight on Midsummer’s night the King of the Fairies dances with his followers on wild thyme beds. It was an ingredient of many magical potions, dating from around 1600, which allowed the user to see fairies. One simple charm was to make a brew of wild thyme tops gathered near the side of a fairy hill plus grass from a fairy throne. It was also an ingredient of the fairy ointment which was applied to the eyes of new-born fairy babies to enable them to see the invisible. Like other fairy flowers, wild thyme is unlucky if brought indoors. It is one of the best herbs used to attract and work with the fairy wildfolk, in offerings, incense and spells.
Illustration Paul Mason
 R.L.Tongue, Somerset Folklore, Folklore Society, 1965
 According to the folklore, good fairies start to come out around the vernal equinox, are very animated by Beltane, and at the peak of their activities by Midsummer. By Halloween, most of the good fairies have disappeared from sight and the bad fairies, such as goblins, rule the winter period.
 W.B.Yeats, Folk and Fairy Tales of the Irish Peasantry, 1888