Saint John's Day on June 24th, the feast day of Saint John the Baptist, was established by the undivided Christian Church in the 4th century A.D., in honor of the birth of the Saint John the Baptist.
Usually, a saint's feast day is celebrated on the day that the saint died. St. John along with the Virgin Mary are the only two saints whose birthdays are celebrated.
In the Gospel of Luke, certain verses imply that John the Baptist was born six months earlier than Jesus. And since it has become a tradition that Jesus was born on December 25th (Christmas Day), Midsummer day, being six months before was taken to be the feast day of St. John the Baptist.
Learn more about how Catholic families can celebrate! Video produced by Catholics All Year: