The Nativity Fast
The cycle of the celebration of the Nativity starts with a fast of forty days that precedes the feast. It is called the Nativity Fast or Advent. For the faithful, it is a time to purify both soul and body to enter properly into and partake of the great spiritual reality of Christ's Coming, much like the preparation for the fast of the Lord's Resurrection, that is Great Lent.
The beginning of the fast on November 15 is not liturgically marked by any hymns, but five days later, on the eve of the Feast of the Presentation of the Theotokos, we hear the first announcement from the nine "Irmoi" of the Christmas Canon: "Christ is born, glorify Him!"
This period includes other special preparatory days announcing the approaching Nativity: St Andrew's Day on November 30; St Nicholas Day on December 6; the Sunday of the Forefathers; and the Sunday of the Fathers.
December 20th begins the Forefeast of the Nativity. The liturgical structure is similar to the Holy Week preceding Pascha.
The Orthodox Church sees the birth of the Son of God as the beginning of the saving ministry which will lead Him, for the sake of man’s salvation, to the ultimate sacrifice of the
Cross.
Presentation of the Theotokos 20th
According to Tradition, the Virgin Mary was taken — presented — by her parents Joachim and Anna into the Jewish Temple in Jerusalem as a young girl, where she lived and served as a Temple virgin until her betrothal to St. Joseph.
One of the earliest sources of this tradition is the non-canonical Protoevangelion of James, also called the Infancy Gospel of James.
Mary was solemnly received by the temple community which was headed by the priest Zacharias, the father of John the Baptist.
She was led to the holy place to become herself the "holy of holies" of God, the living sanctuary and temple of the Divine child who was to be born in her.
The Church also sees this feast as a feast which marks the end of the physical temple in Jerusalem as the dwelling place of God.