The young maid stole through the cottage door, and blushed as she sought the Plant of pow'r;
"Thou silver glow-worm, O lend me thy light, I must gather the mystic St. John's wort tonight,
The wonderful herb, whose leaf will decide, if the coming year shall make me a bride."
In addition to the four great festivals of the Pagan Celtic year, there are four lesser holidays as well: the two solstices, and the two equinoxes. In folklore, these are referred to as the four "quarter-days" of the year, and modern Witches call them the four "Lesser Sabbats," or the four "Low Holidays." The summer solstice is one of them.
Technically, a solstice is an astronomical point and, due to the procession of the equinox, the date may vary by a few days depending on the year. The summer solstice occurs when the sun reaches the Tropic of Cancer, and we experience the longest day and the shortest night of the year. Astrologers know this as the date on which the sun enters the sign of Cancer.
However, since most European peasants were not accomplished at reading an ephemeris or did not live close enough to Salisbury Plain to trot over to Stonehenge and sight down its main avenue, they celebrated the event on a fixed calendar date, June 24th. The slight forward displacement of the traditional date is the result of multitudinous calendrical changes down through the ages. It is analogous to the winter solstice celebration, which is astronomically on or about December 21st, but is celebrated on the traditional date of December 25th, Yule, later adopted by the Christians.
Again, it must be remembered that the Celts reckoned their days from sundown to sundown, so the June 24th festivities actually begin on the previous sundown (our June 23rd). This was Shakespeare's Midsummer Night's Eve. Which brings up another point: our modern calendars are quite misguided in suggesting that "summer begins" on the solstice. According to the old folk calendar, summer begins on May Day and ends on Lammas (August 1st), with the summer solstice, midway between the two, marking midsummer. This makes more logical sense than suggesting that summer begins on the day when the sun's power begins to wane and the days grow shorter.
Just as the Pagan midwinter celebration of Yule was adopted by Christians as Christmas (December 25th), so too the Pagan midsummer celebration was adopted by them as the feast of John the Baptist (June 24th). Occurring 180 degrees apart on the wheel of the year, the midwinter celebration commemorates the birth of Jesus, while the midsummer celebration commemorates the birth of John, the prophet who was born six months before Jesus in order to announce his arrival.
Although modern Witches often refer to the holiday by the rather generic name of Midsummer's Eve, it is more probable that our Pagan ancestors of a few hundred years ago actually used the Christian name for the holiday, St. John's Eve. This is evident from the wealth of folklore that surrounds the summer solstice (i.e. that it is a night especially sacred to the faerie folk) but which is inevitably ascribed to "St. John's Eve," with no mention of the sun's position. It could also be argued that a Coven's claim to antiquity might be judged by what name it gives the holidays. (Incidentally, the name "Litha" for the holiday is a modern usage, possibly based on a Saxon word that means the opposite of Yule. Still, there is little historical justification for its use in this context.) But weren't our Pagan ancestors offended by the use of the name of a Christian saint for a pre-Christian holiday?
Well, to begin with, their theological sensibilities may not have been as finely honed as our own. But secondly, and more importantly, St. John himself was often seen as a rather Pagan figure. He was, after all, called "the Oak King." His connection to the wilderness (from whence "the voice cried out") was often emphasized by the rustic nature of his shrines. Many statues show him as a horned figure (as is also the case with Moses). Christian iconographers mumble embarrassed explanations about "horns of light," while modern Pagans giggle and happily refer to such statues as "Pan the Baptist." And to clench matters, many depictions of John actually show him with the lower torso of a satyr, cloven hooves and all! Obviously, this kind of John the Baptist is more properly a Jack in the Green! Also obvious is that behind the medieval conception of St. John lies a distant, shadowy Pagan deity, perhaps the archetypal Wild Man of the Wood, whose face stares down at us through the foliate masks that adorn so much church architecture. Thus medieval Pagans may have had fewer problems adapting than we might suppose.