Midsummer Day is perfectly described in its name ~ it is the point in the middle of the summer ~ the summer solstice.
The word solstice comes from two words: sol meaning "sun" and sistere meaning "to stand still". There are two solstices during any year: the winter solstice, which occurs around December 21, and the summer solstice, which occurs around the 21st of June (when the Sun enters zero degrees Cancer). Because the earth revolves around the sun in an eliptical, rather than a circular, path, it will be the furtherest from the sun at one point (i.e. the winter solstice), and it will be the closest to the sun at another point (i.e. the summer solstice). During the winter solstice, the day is the shortest of the year; during the summer solstice, the day is the longest of the year.
Midsummer Day, traditionally held as the mid-point of the summer season, occurs on the 24th of June, three days after the actual summer solstice. And Midsummer Eve, the holiday celebrated by non-Christians, occurs, of course, on the eve of the 24th of June. The Catholic Church established 23 June as St. John's Day, a celebration of St. John the Baptist's presumed birthday, in an effort to "christianize" it in the same way that they changed so many other pagan festivals into Christian holidays.
The Celts and other "pagan" peoples held the eve of the summer solstice in reverence. For many cultures, midsummer was the most important holiday of the year. The reason for this was because of the prevalence of Goddess (i.e. Mother Earth) worship. It was believed that the Goddess was pregnant with the bounty of the coming harvest at Midsummer Day.
The name for the summer solstice, or midsummer, varied from place to place; in Scotland it was known as Feill~Sheathain, in England it was known as Alban Heflin, in Wales it was Gathering Day, in Gaul it was the Feast of Epona, in Rome it was Vestalia or Litha, in Greece it was All Couple's Day, and in Scandinavia it was known as Thing-Tide.
The Midsummer Solstice was an important time for practitioners of witchcraft and the occult arts. It was believed that the two solstices and the two equinoxes were times when spells cast would be most potent. And it was believed that the faerie realms were more accessible during the solstices and equinoxes. Therefore during these times, festivals would include dancing and other activities intended to entice the faeries and other inhabitants of the netherworld to come out of hiding.
As with other seasonal festivals, a key feature of Midsummer Eve was the lighting of bonfires on hilltops. The Midsummer Eve bonfire was often created from contributions of wood from every household. In his book, A Philosphical Survey Of The South Of Ireland, In A Series Of Letters, published in 1778, John Watkinson stated:
It is not strange that many Druid remains should still exist; but it is a little extraordinary that some of their customs should still be practised. They annually renew the sacrifices that used to be offered to Apollo, without knowing it. On Midsummer's eve, every eminence, near which is a habitation, blazes with bonfires; and, round these, they carry numerous torches, shouting and dancing, which affords a beautiful sight, and at the same time confirms the observation of Scaliger, En Irlande, ils sont quasi tous Papistes, mais c'est Papaute meslie de Paganisime comme partout.
One salient custom of the festivities regarding the Midsummer Eve bonfire was the action, by many of the youths, of leaping over it. The purpose, apparently, was simply the expression of one's enjoyment of the season.
Another custom on Midsummer Eve was to wrap a wooden wheel with boughs from the birch tree, set it afire, and send it rolling down the hill. It was believed that the wheel represented the Sun, and if the burning wheel continued to roll and remained aflame until it reached the foot of the hill, the Sun would provide a bountiful harvest in the Fall. Unfortunately, if the blazing wheel fell over before reaching the bottom of the hill, or if the fire went out, it portended failing crops.
It was sometimes thought that on Midsummer Eve devils, ghosts and hobgoblins were loosed to fly about attempting to bring harm to anyone out of doors on Midsummer Eve. So for that reason, when a person left the safety of the bonfire, a firebrand would be taken from the fire, in hopes that it would lend them safety until his/her home could be reached.
The lintels of house's doors were often decorated with boughs of birch, fennel, St. John's Wort, orpine, white lilies and other wild flowers. The birch tree, known as beth to the early Celts, was called the lady of the forest and was a symbol of fertility and birth. The birch tree was associated with the Moon, as a feminine force. In the far north, the birch tree was associated with the Norse god, Thor, a god of fertility. Fennel was believed to promote longevity and prevent obesity. Its inclusion in wreaths and boughs hung over doorways on Midsummer Eve was in the hopes of preventing ghosts from enterring the house. St. John's Wort was, like fennel, believed to prevent evil spirits from enterring a house, and it also helped to dispel bad dreams from the people who slept in the house protected by it. Orpine is a type of sedum and was used to grant long life to the members of a household; its name means long-lived because it can live a while after it is dug up and removed from the soil. The white lily was a symbol of innocence and purity.
Boughs of birch, which had been blessed on Midsummer Eve would be hung over the doors of barns to protect the cattle from witchcraft
As noted above, in Wales Midsummer was known as Gathering Day. The Celts in the western reaches of Britain believed that this was the most opportune time of the year to gather certain herbs and sacred plants such as mistletoe and vervain. Mistletoe had to be cut by a Druid with a golden sickle on Midsummer Eve in order to be most effective for healing and divination. The common man would use an ordinary knife to cut his herbs on Midsummer Eve, but often it was an ordinary knife that was reserved for use only on Midsummer Eve. Whether cut by a Druid or a common man, the herbs and sacred plants that were gathered on Midsummer Eve had to be caught, as they fell, in a piece of cloth. If they touched the ground when cut, their potent force would dissipate into the earth.
A superstition that came down throught the ages was that fern-seed had to be gathered on Midsummer Eve. It was believed that fern-seed had great magical powers; the only time that it could be gathered was on Midsummer Eve. If it were gathered at any other time of the year, the person would arrive home to find the bag or box into which it had been placed to be empty.
Young ladies, on Midsummer Eve around midnight, would lay a clean cloth on a table, on which she would then place bread, cheese and wine. She would open the door and then sit down beside the table. From the passersby, the man whom she was to marry would be the one who would take the time to step in through the open door and sit down to eat with her.
To dream of the man who she would marry, a young lady would go into a field around midnight on Midsummer Eve, and getting down on her hands and knees, she would search under the plantain plants for a coal. If she were lucky enough to find one, she would place it under her pillow when she went to sleep that night. Her dreams would show her her future husband.