The first day of summer is usually about June 21st. This is known as the summer solstice; in the Northern hemisphere we get the longest period of daylight and shortest night all year. The daylight hours had been increasing for the past six months, and from that point would decrease for six months.
According to Luke 1:24-38, John was conceived about six months before Jesus; they were half a year apart in age. The Church celebrates the birth of Christ on the accepted (and likely historically-accurate) date of December 25th. And we celebrate the birth of John on June 24th, about six months earlier.
John said of Jesus and himself, “He must increase; I must decrease” (John 3:30). The Northern hemisphere responds accordingly, shortening the length of days after John’s birthday for the following six months. Then, on Christmas Day, when “The true light that enlightens every man [came] into the world” (John 1:9), the days start getting longer again.
As Psalm 19 opens, “The heavens declare the glory of God!” It is more than convenient that John and Jesus were born at such noteworthy points in the year, for God who made them “for signs and for seasons and for days and years” (Genesis 1:14), is pleased to proclaim the glory of his eternal Son by all means available!