The Nativity Fast is the forty-day preparation before the feast of the Nativity of Christ, the Orthodox Advent. Beginning in mid-November, it readies the faithful to receive the Incarnate God not amid distraction but with a heart made ready, as the world outside grows loud with a different kind of Christmas.
It is a gentler fast than Great Lent, brightening as the feast draws near, with the great feast of the Entry of the Theotokos falling within it. From the feast of St Spyridon the Church begins to sing the hymns of the coming Nativity, and the fast turns from restraint to expectation, the long Advent of a people waiting for their God.
As always, the fast is a means and not the end. Its purpose is the readied heart, and its keeping is fitted to each person with their priest. The point of the forty days is that when the feast comes, one is not too full to receive it.