The Feast of the Entrance into the Temple of Our Most Holy Lady the Theotokos and Ever-Virgin Mary is celebrated on November 21 each year. The Feast commemorates when as a young child, the Virgin Mary entered the Temple in Jerusalem.
The birth and early life of the Virgin Mary is not recorded in the Gospels or other books of the New Testament, however this information can be found in a work dating from the second century known as the Book of James or Protevangelion.
When Mary was three years old, Joachim and Anna decided that the time had come to fulfill their promise and to offer her to the Lord. Joachim gathered the young girls of the neighborhood to form an escort, and he made them go in front of Mary, carrying torches. Captivated by the torches, the young child followed joyfully to the Temple, not once looking back at her parents nor weeping as she was parted from them.
The holy Virgin ran toward the Temple, overtaking her attendant maidens and threw herself into the arms of the High Priest Zacharias, who was waiting for her at the gate of the Temple with the elders. Zacharias blessed her saying, "It is in you that He has glorified your name in every generation. It is in you that He will reveal the Redemption that He has prepared for His people in the last days."
Then, Zacharias brought the child into the Holy of Holies—a place where only the High Priest was permitted to enter once a year on the Day of Atonement. He placed her on the steps of the altar, and the grace of the Lord descended upon her. She arose and expressed her joy in a dance as wonder seized all who saw this happen.
The Virgin Mary dwelt in the Temple for nine years until, reaching an age for marriage, she was taken from the Temple by the priests and elders and entrusted to Joseph as the guardian of her virginity.
The Entrance of the Theotokos into the Temple signifies her total dedication to God and her readiness for her future vocation as the Mother of the Incarnate Lord. This is a feast of anticipation. As honor is shown to Mary, the faithful are called to look forward to the Incarnation of Christ, celebrated in a little more than a month by the Feast of the Nativity on December 25.
Continue reading the full article on the website of the Greek Archdiocese, which includes an explanation of the icon of the Feast, the prescribed Scripture readings, and numerous additional resources...
The Monasticism of the Panagia
Monastics associate this great feast with the mystery of the monastic state.
By Archimandrite Porphyrios, Abbot of the Sacred Monastery of the Honorable Forerunner in Beroia
The feast of the Entrance of the Theotokos is called by my fathers on the Holy Mountain "the monasticism of the Panagia" (η καλογερική της Παναγίας).
Elder Gelasios loved her very much and would say that Papa-Thanasi, his Elder, the holy abbot of Gregoriou, honored her very much. And on one such day Elder Gelasios saw my Elder co-liturgizing with Saint James the Brother of God, who preserved this tradition.
The Panagia enters the Holy of Holies. Her parents, our grandparents Saint Joachim and Saint Anna, fulfill their vow. Many young girls of Jerusalem hold lit lamps and open a red cloth, like the bridal cloth that would be put on the backs of couples in olden times at the time of the celebration of the sacred mystery of Marriage.
How many symbols! And how many centuries has the world waited for this hour! For the Ever-Virgin to enter into a prepared place, where male high priests trembled to approach and enter once every annual cycle.
It is impossible for the mind to comprehend, and many "scientists" deny this feast in particular.
But of course they will deny it, since they live by logic-bound thought. And wherever "irrational" logic enters, the supra-rational Mind, our Triune God, departs.
And the archangel of joy, of the gospel and of hope serves her. What a fearsome mystery!
The Athonites associate this great feast with monasticism, with the mystery of the monastic state.
Certainly the place of our holy of holies is a place deep in our hearts. It is where the mixing of the created and the Uncreated takes place, in complete light.
When the heart is completely empty, it is renewed by the Young Child, our Lord Jesus Christ, in complete and salvific light.
Empty is the sacred sanctuary of the Temple of Jerusalem, and enthroned there is little Mary, the child of Nazareth.
The monk, the beautiful old man, is he who enters his own sanctuary, when he forgets and leaves. And, what a mystery is this, when he makes the little Virgin turn back, he gets excited by the light and the red color that shines in the light, and he rushes forward again, to his sacred bridal chamber, his unique and only place of rest, deep within the heart. And there are tears.
The monasticism of the Ever-Virgin! The great high priest and prophet grants her entrance. Yes, he is even a prophet, for him to know how many centuries the prophets before him prophesied of this moment. The message of the prophets, is the Lady Theotokos. It is spoken about also in the troparion. And the renewal of the faithful; we, that is, who want to be faithful to the faith and faithful to the Church, we are reconstructing-changing, and every moment we become, if we want, new again: we are filled with her light and believe correctly in the Church.
As an Athonite, how much did Saint Gregory [Palamas], the Theologian of the Uncreated Light of our God, love this feast? His homily on it is unlike any other. Pure excitement, waves of grace and an ocean of love. 
My brother, if you find yourself at Hilandari, our beloved, lift up your eyes above the right choir, and dance with the monks; see the company of little Mary and her parents and the high priest. Look, and be excited. And you also will live, even for a short time, inside the holy of holies of the cosmos, this false world, on sacred Athos, her Garden, the Holy Mountain.
And remember, the Lady Theotokos is our renewal.
 Read St Gregory Palamas' Homily on the Entrance of the Theotokos here. Order the collected Sermons of St Gregory Palamas on the Mother of God here.
Source: Translated by John Sanidopoulos. Posted at Mystagogy.