John the Baptist (c. 1st century BC – c. AD 30) is said to have been an itinerant preacher active in the area of Jordan River in the early 1st century AD.[19] He is also known as John the Forerunner in Christianity, John the Immerser in some Baptist Christian traditions,[20] and Prophet Yahya in Islam. He is sometimes alternatively referred to as John the Baptizer. (From Wikipedia)
The Feast of Saint John closely coincides with the June solstice, also referred to as Midsummer in the Northern Hemisphere. The Christian holy day is fixed at 24 June; but in most countries festivities are mostly held the night before, on Saint John's Eve. "In England, 'Saint John's Tide' is combined with a midsummer celebration.
Instead of the date of the summer solstice, they chose June 24. This may be because of the Baptist's own words, 'He must increase, but I must decrease' (John 3:30). John was, of course, referring to Jesus. John's day comes at the time when the sun is beginning to decrease..." (From Wikipedia)