March 25 falls exactly nine months before Christmas — by ancient reckoning, the day of Christ's conception in the womb of Mary. On this feast the Church remembers Gabriel's annunciation and Mary's fiat: "Let it be done unto me according to thy word."
The early Christians read deep meaning into the date. They held that March 25 was also the day of the world's creation, and the day of Christ's death — that all the great works of God converged on the spring equinox. Whether or not the dating is historical, the theology is clear: in Mary's womb, on a date the Church has kept since the fourth century, the eternal Word took flesh.