The Christmas celebration as we know it today had its real beginnings with Constantine's reinvention of the Christian church after 313 A.D. (For details regarding Constantine's considerable influence on Christian practices, teachings, and institutions, see http://www.bibletexts.com/terms/313ad.htm.)

In his excellent article on the origins of the Christmas celebration, Hendrik F. Stander (Encyclopedia of Early Christianity, Second Edition, edited by Everett Ferguson, NY: Garland Publishing, 1998, pages 249-250):

CHRISTMAS. From Old English "Mass of Christ," festival on December 25, celebrating the birth of Christ. This festival was not generally established before the end of the fourth century. The early Christians were far more concerned with the death and resurrection of Jesus, celebrated during Easter, than with his birth. Moreover, Origen said that Christians should not celebrate birthdays because it was a pagan custom, adhered to by unrighteous people, such as Pharaoh and Herod (Comm. in Mt. 10.22)...

The true roots of what we know as Christmas lie in a feast celebrating the baptism of Christ on January 6. The heretical Basilideans taught that the divine Christ first appeared on earth at the baptism of Jesus and was then temporarily united with the human Jesus. The festival on January 6 was accordingly called Epiphany ("Appearance"). The orthodox church regarded Christ's birth as the real appearing of Christ upon the earth, and thus January 6 came to mark both the baptism and the nativity of Christ.

The Council of Nicaea (325) condemned the doctrine that God himself did not become incarnate in Jesus at birth. This Christological dogma probably caused the festival of Christ's birth to be separated from the heretical custom of commemorating Christ's "appearance" at baptism. The festival of the birth of Christ ... was then transferred to December 25, to counter a pagan festival held that day in honor of Sol Invictus ("The Invincible Sun").

The nativity festival was probably separated from Epiphany in Rome between 325 and 354. The first evidence of the celebration of Christ's birth on December 25 comes from Rome in the year 336, although the older Epiphany festival continued to exist for some time in its original form.

...The strongest opposition, however, came from Christians in Jerusalem, and it was not until the middle of the sixth century that Palestinian Christians accepted the festival of December. The Armenian church continues even today to commemorate Christ's birth on January 6.

...One theory for placing Christmas on 25 December suggests that the motivation for a Christian observance came from the pagan birthfeast of the unconquered sun (dies natalis solis invicti) which the Roman emperor Aurelian, in 274, ordered to be held annually throughout the empire on the day of the winter solstice, determined then to be 25 December. According to the 'apologetics' or 'history of religions' hypothesis, the church at Rome adopted the day of pagan festivities for its own purposes, providing a Christian alternative by celebrating instead the birth of the true 'sun of righteousness'; the heathen content of the festival was reinterpreted and its date retained...

The festival of Christmas spread quickly from the 4th century onwards. It allowed for a liturgical counterpart to the theological conclusion reached at the Council of Nicaea which, in condemning Arianism, affirmed that Jesus Christ was the eternal and only-begotten Son of the Father, humanly born of Mary (recognized by the Council of Ephesus as the 'God-bearer', Theotokos)...

Addressing pagans:

On your day of gladness, we [Christians] neither cover our doorposts with wreaths, nor intrude upon the day with lamps. At the call of public festivity, you consider it a proper thing to decorate your house like some new brothel. ... We are accused of a lower sacrilege because we do not celebrate along with you the holidays of the Caesars in a manner forbidden alike by modesty, decency, and purity. [197 A.D., ANF 3:44]

The Roman traitors clad their doorposts with green and branching laurels. They smoked up their porches with lofty and brilliant lamps. [197 A.D., ANF 3:44]

Addressing Christians:

What less of a defilement does he incur on that ground than does a business ... that is publicly consecrated to an idol? The Minervalia are a much Minerva's as the Saturnalia is Saturn's. Yes, it is Saturn's day, which much necessarily be celebrated even by little slaves at the time of the Saturnalia. Likewise, New Year's gifts must be caught at. The Septimontium must be kept. And all the presents of Midwinter and the Feast of Dear Kinsmanship must be exacted. The schools must be wreathed with flowers.... The same thing takes place on an idol's birthday. Every ceremony of the devil is frequented. Who will think that these things are benefiting to a Christian teacher? [200 A.D., ANF 3:66]

The Saturnalia, New Year, Midwinter festivals, and Matronalia are frequented by us! Presents come and go! There are New Year's gifts! Games join their noise! Banquets join their din! The pagans are more faithful to their own sect... For, even if they had known them, they would not have shared the Lord's Day or Pentecost with us. For they would fear lest they would appear to be Christians. Yet, we are not apprehensive that we might appear to be pagans! [200 A.D., ANF 3:69]

The following are the biblical texts from the ESV that reference the birth of Jesus:

• Mat 1:18-25 - The birth of Jesus

18 Now the birth of Jesus Christ took place in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child from the Holy Spirit. 19 And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly. 20 But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, "Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit. 21 She will bear a son, and you shall call his name Jesus, for he will save his people from their sins." 22 All this took place to fulfill what the Lord had spoken by the prophet: 23 "Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel" (which means, God with us). 24 When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, 25 but knew her not until she had given birth to a son. And he called his name Jesus.

* See commentary at http://www.bibletexts.com/verses/v-mat.htm#1.

• Mat 2:1-12 - The visit of the Magi from the East to the infant Jesus

1 Now after Jesus was born in Bethlehem of Judea in the days of Herod the king, behold, wise men from the east came to Jerusalem, 2 saying, "Where is he who has been born king of the Jews? For we saw his star when it rose and have come to worship him." 3 When Herod the king heard this, he was troubled, and all Jerusalem with him; 4 and assembling all the chief priests and scribes of the people, he inquired of them where the Christ was to be born. 5 They told him, "In Bethlehem of Judea, for so it is written by the prophet: 6 "' And you, O Bethlehem, in the land of Judah, are by no means least among the rulers of Judah; for from you shall come a ruler who will shepherd my people Israel.'" 7 Then Herod summoned the wise men secretly and ascertained from them what time the star had appeared. 8 And he sent them to Bethlehem, saying, "Go and search diligently for the child, and when you have found him, bring me word, that I too may come and worship him." 9 After listening to the king, they went on their way. And behold, the star that they had seen when it rose went before them until it came to rest over the place where the child was. 10 When they saw the star, they rejoiced exceedingly with great joy. 11 And going into the house they saw the child with Mary his mother, and they fell down and worshiped him. Then, opening their treasures, they offered him gifts, gold and frankincense and myrrh. 12 And being warned in a dream not to return to Herod, they departed to their own country by another way.

* See commentary at http://www.bibletexts.com/verses/v-mat.htm#2.

• Luk 1:26-56 - The birth of Jesus is announced

26 In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, 27 to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin's name was Mary. 28 And he came to her and said, "Greetings, O favored one, the Lord is with you!" 29 But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. 30 And the angel said to her, "Do not be afraid, Mary, for you have found favor with God. 31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. 32 He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, 33 and he will reign over the house of Jacob forever, and of his kingdom there will be no end." 34 And Mary said to the angel, "How will this be, since I am a virgin?" 35 And the angel answered her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy- the Son of God. 36 And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren. 37 For nothing will be impossible with God." 38 And Mary said, "Behold, I am the servant of the Lord; let it be to me according to your word." And the angel departed from her. 39 In those days Mary arose and went with haste into the hill country, to a town in Judah, 40 and she entered the house of Zechariah and greeted Elizabeth. 41 And when Elizabeth heard the greeting of Mary, the baby leaped in her womb. And Elizabeth was filled with the Holy Spirit, 42 and she exclaimed with a loud cry, "Blessed are you among women, and blessed is the fruit of your womb! 43 And why is this granted to me that the mother of my Lord should come to me? 44 For behold, when the sound of your greeting came to my ears, the baby in my womb leaped for joy. 45 And blessed is she who believed that there would be a fulfillment of what was spoken to her from the Lord." 46 And Mary said, "My soul magnifies the Lord, 47 and my spirit rejoices in God my Savior, 48 for he has looked on the humble estate of his servant. For behold, from now on all generations will call me blessed; 49 for he who is mighty has done great things for me, and holy is his name. 50 And his mercy is for those who fear him from generation to generation. 51 He has shown strength with his arm; he has scattered the proud in the thoughts of their hearts; 52 he has brought down the mighty from their thrones and exalted those of humble estate; 53 he has filled the hungry with good things, and the rich he has sent empty away. 54 He has helped his servant Israel, in remembrance of his mercy, 55 as he spoke to our fathers, to Abraham and to his offspring forever." 56 And Mary remained with her about three months and returned to her home.

* See commentary at http://www.bibletexts.com/verses/v-luk.htm#1.

• Luk 2:1-7 - The birth of Jesus

1 In those days a decree went out from Caesar Augustus that all the world should be registered. 2 This was the first registration when Quirinius was governor of Syria. 3 And all went to be registered, each to his own town. 4 And Joseph also went up from Galilee, from the town of Nazareth, to Judea, to the city of David, which is called Bethlehem, because he was of the house and lineage of David, 5 to be registered with Mary, his betrothed, who was with child. 6 And while they were there, the time came for her to give birth. 7 And she gave birth to her firstborn son and wrapped him in swaddling cloths and laid him in a manger, because there was no place for them in the inn.

* See commentary at http://www.bibletexts.com/verses/v-luk.htm#2.

• Luk 2:8-20 - The visit of the shepherds to the infant Jesus

8 And in the same region there were shepherds out in the field, keeping watch over their flock by night. 9 And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with fear. 10 And the angel said to them, "Fear not, for behold, I bring you good news of a great joy that will be for all the people. 11 For unto you is born this day in the city of David a Savior, who is Christ the Lord. 12 And this will be a sign for you: you will find a baby wrapped in swaddling cloths and lying in a manger." 13 And suddenly there was with the angel a multitude of the heavenly host praising God and saying, 14 "Glory to God in the highest, and on earth peace among those with whom he is pleased!" 15 When the angels went away from them into heaven, the shepherds said to one another, "Let us go over to Bethlehem and see this thing that has happened, which the Lord has made known to us." 16 And they went with haste and found Mary and Joseph, and the baby lying in a manger. 17 And when they saw it, they made known the saying that had been told them concerning this child. 18 And all who heard it wondered at what the shepherds told them. 19 But Mary treasured up all these things, pondering them in her heart. 20 And the shepherds returned, glorifying and praising God for all they had heard and seen, as it had been told them.

* See commentary at http://www.bibletexts.com/verses/v-luk.htm#2.

• Gal 4:3-6 - Christ Jesus born of woman

  3 In the same way we also, when we were children, were enslaved to the elementary principles of the world. 4 But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, 5 to redeem those who were under the law, so that we might receive adoption as sons. 6 And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!"

  * See commentary at http://www.bibletexts.com/verses/v-gal.htm#4.

• See also http://www.bibletexts.com/sh/by-phrase.htm#christmas