The question would naturally arise as to why the celebration falls on June 24 rather than June 25 if the date is to be precisely six months before Christmas.
It has often been claimed that the Church authorities wanted to Christianize the pagan solstice celebrations and for this reason advanced Saint John's feast as a substitute.
This explanation is questionable because in the Middle Ages the solstice took place around the middle of June due to the inaccuracy of the Julian calendar.
It was only in 1582, through the Gregorian calendar reform, that the solstice returned to June 21 as it had been in the fourth century.

Therefore, a more likely reason why the festival falls on June 24 lies in the Roman way of counting, which proceeded backward from the Kalends (first day) of the succeeding month.
Christmas was "the eighth day before the Kalends of January" (Octavo Kalendas Januarii).
Consequently, Saint John's Nativity was put on the "eighth day before the Kalends of July."
However, since June has only thirty days, in our present (Germanic) way of counting, the feast falls on June 24.

Nevertheless, the fact of the feast falling around the time of the solstice is considered by many to be significant, recalling the words of John the Baptist with regard to Jesus: "He must increase, but I must decrease" (John 3:30).
The question would naturally arise as to why the celebration falls on June 24 rather than June 25 if the date is to be precisely six months before Christmas. It has often been claimed that the Church authorities wanted to Christianize the pagan solstice celebrations and for this reason advanced Saint John's feast as a substitute. This explanation is questionable because in the Middle Ages the solstice took place around the middle of June due to the inaccuracy of the Julian calendar. It was only in 1582, through the Gregorian calendar reform, that the solstice returned to June 21 as it had been in the fourth century.
Therefore, a more likely reason why the festival falls on June 24 lies in the Roman way of counting, which proceeded backward from the Kalends (first day) of the succeeding month. Christmas was "the eighth day before the Kalends of January" (Octavo Kalendas Januarii). Consequently, Saint John's Nativity was put on the "eighth day before the Kalends of July." However, since June has only thirty days, in our present (Germanic) way of counting, the feast falls on June 24.
Nevertheless, the fact of the feast falling around the time of the solstice is considered by many to be significant, recalling the words of John the Baptist with regard to Jesus: "He must increase, but I must decrease" (John 3:30).
Nativity of Saint John the Baptist