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Saint John's Day, the feast day of Saint John the Baptist, was established by the undivided Christian Church in the 4th century AD, in honour of the birth of the Saint John the Baptist, which the Gospel of Luke records as being sixth months before Jesus. As the Western Christian Churches mark the birth of Jesus on December 25, Christmas, the Feast of Saint John (Saint John's Day) was established at midsummer, exactly sixth months before the former feast. 
4th century AD