The Orthodox Church prepares Her children that they might worthily greet the Nativity of Christ by means of the forty-day Nativity Fast, which lasts from 15th November until 24th December (on the Church calendar). In addition to the reasons which are generally known, the Nativity Fast is observed by Orthodox Christians so that they might honour the sufferings and afflictions which, immediately before the all-holy event of the Nativity, the Most Holy Theotokos was made to endure from the scribes and Pharisees.
Sacred Tradition tells that some little time before the Righteous Joseph and the All-holy Virgin set off for Bethlehem, the following testing befell them. A certain scribe, Ananias, visited their house and saw that the Virgin was pregnant. He was scandalized by this, and he went to the high priest and the whole council of the Jews and said, "Joseph the Carpenter, who is considered righteous, has behaved unlawfully. Secretly he has corrupted and defiled the Virgin, who was entrusted to him from the Temple for safe-keeping. And now she is pregnant." Then servants of the high priest were sent to Joseph's house and finding the Virgin indeed pregnant, they took her and Joseph and brought them before the high priest, who began to accuse and shame the the holy Maiden.
However, in her deep affliction, the Virgin responded weeping:"As the Lord lives, my God, He is my Witness that I am pure and have not known a man." The high priest then accused the righteous Joseph, but with an oath he affirmed that he was not guilty of this sin. The high priest did not believe them, and subjected them to the testing that was used at that time (see Numbers 5:19-31). However, this testing only served to demonstrate the innocence of Most Holy Virgin and of Joseph. All those in the council wondered at this, for they were unable to understand how the Virgin could be pregnant and at the same time innocent.Nonetheless the high priest let the holy couple go in peace. The Righteous Joseph, taking the Virgin Mary, returned to his home rejoicing and praising God (see "The service for the Feast of the Nativity with appendices" published in Russian by HolyTrinity Monastery, 1984).
But this was not the end of the things which the Most Holy Theotokos had to endure. With Joseph she shared the difficulty of the three day journey from Nazareth to Bethlehem. In Bethlehem no place could be found for the Immaculate Virgin either in the inn or in any of the houses, and the day was declining into evening. She and the Righteous Joseph had to find shelter in a cave which was used as a stable for domestic cattle. In this wretched shelter, the most blessed Virgin continued in prayer and in Godly thoughts. Here it was that she gave birth, without pain, to our Lord Jesus Christ, the Saviour of the world.
From what has been told above, we can see that the days immediately before the Nativity, were for the Most Holy Theotokos not days of peace and comfort. During these days she had to endure various afflictions and temptations, but through all this she remained steadfast in prayer and in thoughts upon God.
The Holy Church suggests to her children that they participate, even though it be to the smallest degree, in the All-holy Theotokos' struggle, that they constrain their flesh during the Nativity Fast, and that they nourish their souls with prayer. However, the Church also gives us advance notice that a merely outward fast is not sufficient. We must also keep an inner fast, which consists in restraining and estranging ourselves from evil, from falsehood, anger, vanity and the other vices. It is essential during the fast, as it is always, to manifest love for our neighbours, to do acts of mercy, increasing our help for those in need and in affliction. Then our fast will be a real one, and not hypocritical; it will be truly pleasing unto God and we shall approach the radiant festival of Christ's Nativity with joy.
Translated from "Pravoslavnaya Rus" 1/14th November 1999