Today the feast of the Nativity of Saint John the Baptist takes precedence over the Fifth Sunday after Pentecost, which is commemorated in the Mass and the Breviary.
Ordinarily the Church observes the day of a saint's death as his feast, calling it his dies natalis, or the day of his birth into heaven. To this rule there are two notable exceptions, the birthdays of Blessed Mary and of St. John the Baptist. All other persons were stained with original sin at birth, hence, were displeasing to God. But Mary, already in the first moment of her existence, was free from original sin, and John was cleansed of original sin in the womb of his mother. This is the dogmatic justification for today's feast.
In the breviary St. Augustine explains that the reason the day of St. John’s birth is sacred, whereas in the case of other saints, only the final day of their lives is honored, is that the Lord willed to announce to men His own coming through the Baptist, lest if He appeared suddenly, they would fail to recognize Him. John represented the Old Covenant and the Law. Therefore he preceded the Redeemer, even as the Law preceded and heralded the new dispensation of grace.
The birth of Jesus is observed on December 25 at the time of the winter solstice, while the birth of His forerunner is observed six months earlier at the time of the summer solstice. Christmas is a "light" feast; the same is true today. The popular custom centering about "St. John's Fire" [bonfires made on the vigil of the feast] stems from soundest Christian dogma and could well be given renewed attention. St. John's Fire symbolizes Christ the Light; John was a lamp that burned and shone. We Christians should be the light of the world.
Source: Rev. Fr. P. Parsch, 1958, adapted and abridged.