Ordinarily the Church observes the day of a saint’s death as his feast, because that day marks his entrance into heaven. To this rule there are two notable exceptions, the birthdays of Blessed Mary and of St. John the Baptist. In the breviary, St. Augustine explains the reason for today’s observance in the following words:

“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.”