The Christmas Story that we generally hear every season is not found in the Bible.  It is a mixture of two separate and different stories we find In the Gospel of Matthew and Luke.  We normally hear about an angel coming to the Virgin Mary telling her that she is going to have a child though a virgin who will be the savior. Joseph and Mary go to Bethlehem because of a census and can find nowhere to stay. So they stay in a manger. Jesus is born and a star appears. Magi (wise men) come and worship the baby Jesus. An angel warns Joseph and Mary that Herod is afraid that Jesus will take his place as King and that they should go to Egypt. Once Herod had died they return not to their home Bethlehem since Herod's son was now in power but to Nazareth.

However, the only thing that the two gospels Matthew and Luke have in common are that Jesus was known to be from Nazareth but the Messiah was supposed to be from Bethlehem, the town of King David. The Messiah was supposed to be the descendent of David.  

In Matthew Joseph and Mary live in Bethlehem. So Jesus is born there.  They end up in Nazareth because when they return from hiding in Egypt they fear Herod's son.  The Magi come to Joseph and Mary's house not a manger.

In Luke Joseph and Mary live in Nazareth and go to Bethlehem for the census. So Jesus gets born there. They then return to Nazareth. There are no Magi.

The two gospels solve the issue of Jesus as being known from Nazareth but the Messiah (Christ) supposedly being from Bethlehem in two different and fabricated ways.  That this was an issue can be seen in how the Gospel of John handles the problem:

John 7:41-42 (New King James Version)

41 Others said, “This is the Christ.”
But some said, “Will the Christ come out of Galilee? 42 Has not the Scripture said that the Christ comes from the seed of David and from the town of Bethlehem, where David was?”

Nazareth was in Galilee.

Matthew quotes Isaiah 7:14 not from the original Hebrew but from the Septuagint (Greek version of the Old Testament).  The passage was a sign Isaiah was giving not to future people but the King of Isaiah's time, Ahaz.

The Hebrew reads: Assuredly, my Lord will give you a sign of His own accord! Look, the young woman is with child and about to give birth to a son. Let her name him Immanuel.

Wikipedia:

Verb tense and time of pregnancy

Thus Jews understand that God indicated he was sending a "sign" in the days of Ahaz (who lived many centuries before Jesus). Isaiah wanted King Ahaz to wait for God to give him support in this troublesome time instead of making alliances with Assyria.[4]

Moreover, Jews[who?] observe that there is no indication that Immanuel will be the Messiah, whatever the timing of his birth.

Isaiah's original Hebrew, reads (transliterated): Hinneh ha-almah harah ve-yeldeth ben ve-karath shem-o immanuel. The word almah is part of the Hebrew phrase ha-almah hara, meaning "the almah is pregnant." Since the Hebrew imperfect tense is used, it is argued by some, such as rabbi Alfred J. Kolatch (1985)[5] that the young woman was already pregnant and hence not a virgin. As such, the verse cannot be cited as a prediction of the future. However the Jewish Publication Society (JPS) (1917) version reads "Therefore the L-rd Himself shall give you a sign: behold, the young woman shall conceive, and bear a son, and shall call his name Immanuel."

The Jewish tradition has never considered Isaiah 7:14 as a messianic prophecy. Jewish scholars argue that this is a Christian misinterpretation.

Jewish objections to Jesus as the fulfillment of Isaiah 7:14

Jewish objections to Jesus as the fulfillment of Isaiah 7:14:[9][10]

    * If Christians claim that the virgin birth of Isaiah 7:14 was fulfilled twice, who then was the first virgin having a baby boy in 732 BC? If they insist that the word ha'almah can only mean virgin, are they claiming that Mary was not the first and only virgin to conceive and give birth to a child?
    * What does the "butter and honey" refer to?(Isa 7:22)
    * Why is Jesus, who was sinless from birth in the traditional Christian understanding, described as having to learn to refuse the evil and choose the good?[Isa. 7:15-16]
    * What age did the baby Jesus mature?
    * Which were the two kingdoms during Jesus' lifetime that were abandoned?[Isa. 7:16]
    * Who dreaded the Kingdom of Israel during the first century AD when there had not been a Kingdom of Israel in existence since the seventh century BC?
    * Jesus' name is Jesus; his name is not Emmanuel.

The following verses in Isaiah read:

14 Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel. 15 He shall eat curds and honey when he knows how to refuse the evil and choose the good. 16 For before the boy knows how to refuse the evil and choose the good, the land whose two kings you dread will be deserted. 17 The Lord will bring upon you and upon your people and upon your father's house such days as have not come since the day that Ephraim departed from Judah — the king of Assyria.”

The kings Ahaz feared were defeated in his life time as Isaiah and these verses said (at least according to the original story).

The author of the Gospel of Matthew does not know Hebrew and uses the Septuagint which uses parthenos ( translating young woman as virgin ). Thus a statement by Isaiah to Ahaz about their own period of time is transformed into a prophecy about a virgin birth that is to occur centuries later.

On a side note the author of Matthew also misunderstands scripture using the Septuagint to quote alleged prophecy about Jesus entering Jerusalem (the triumphal entry).  He has Jesus straddle two animals, a donkey AND its colt, because he misunderstands the Greek word kai for "and" instead of as "even", a mistake the other gospels did not make.

Were the reasons such stories written to compete with the other miraculous births of Greek heroes and gods?  Or did it simply happen because someone could not understand the Greek version of Isaiah and its context? Parthenos can refer to the young age of a maiden not necessarily her sexual status.  This is a very early example of Christian "cherry picking if you ask me" in more ways than one.