It's the feast of the Nativity of Mary! Which means, of course, that it's her birthday! And, as you have probably guessed by now...I love a birthday. They are full of happiness, just knowing that's the day they were brought into the world. What's not to celebrate about that, ever? This feast of the Nativity of Mary "is a commemoration of that happy and joyful day on which the ever-blessed virgin Mother of God first saw the light of day."
This feast is one of the few that celebrate the birth of a holy person. Most feasts celebrate the death of the holy person...which sounds really weird, but the idea is that the day of death is the day they enter heaven, which is something worth celebrating on their behalf. But this is one of the three days on the church calendar that celebrates a birth-day; only John the Baptist and Jesus himself get this privilege. So this is an extra special honor, signifying her special role in our lives and her relation to Christ.
All of us are born and have many chances to turn to God or away, and really, you can't ever know for sure until you get there (by which I mean, you die). So, for most of us regular Joes, we hope to keep turning toward God all along the way and we only mark and celebrate the death of the holiest, mostly saintly folks....and not their births; because they stayed the course, or turned to it for good. But for Mary, we get to celebrate her birth because God himself made her for himself, to be his entrance as a man to this world. Now that's cool.
"We pray Thee, O Lord, grant to Thy servants the gift of heavenly grace: as the childbearing of the blessed Virgin was the beginning of our salvation, so may the devout celebration of her Nativity accord us an increase of peace."