"Apart from the most holy solemnity commemorating our Savior's birth, the Church keeps the birthday of no other person except that of John the Baptist. (The feasts of the Immaculate Conception and of the Nativity of the Blessed Virgin had not yet been introduced.) In the case of other saints or of God's chosen ones, the Church, as you know, solemnizes the day on which they were reborn to everlasting beatitude after ending the trials of this life and gloriously triumphing over the world.
"For all these the final day of their lives, the day on which they completed their earthly service is honored. But for John the day of his birth, the day on which he began this mortal life is likewise sacred. The reason for this is, of course, 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."
In other words, today's feast anticipates the feast of Christmas. Taking an overall view, we keep during the course of the year only two mysteries, that of Christ's Incarnation and that of His Redemption. The Redemption mystery is the greater of the two; the Incarnation touches the human heart more directly. To the Redemption mystery the entire Easter season is devoted, from Septuagesima until Pentecost; and likewise every Sunday of the year, because Sunday is Easter in miniature.
The Christmas season has for its object the mystery of God-become-Man, to which there is reference only now and then during the remaining part of the year, e.g., on Marian feasts, especially that of the Annunciation (March 25) and today's feast in honor of the Baptist. In a sense, then, we are celebrating Christ's incarnation today. 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" 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.