This feast day marks the birth of John the Baptist, cousin of Jesus, three months after Mary visited her 6 month pregnant cousin Elizabeth. Exactly 6 months later on December 25 we mark the birth of Jesus. John, a prophet, recognized Jesus even in his mother's womb and predicted when he met Jesus as an adult that Jesus would increase and John himself would decrease. These events and predictions are marked in the heavens in that the sun reaching its highest point in the sky on the summer solstice (June 25 on the old calendar) begins its journey downward in the sky until the winter solstice (Dec. 25 on the old calendar) when the hours of daylight increase. Thus, days get shorter after the feast of St. John the Baptist but get longer after the feast of the Nativity of our Lord.
Also note: St. John is the patron saint of Canada so all Canadians should attend services on this day.