Primitive Christianity regarded the birth of Jesus as a significant moment. It was important in the understanding of his personhood and the biblical stories of the events as told by Matthew and Luke in the new testament. It is also interesting to note that Jesus' birth was not observed by the church until the 4th century and then it was chosen to counter some pagan festivals.
Santa Claus has quite a history. An ancient Teutonic festival had a person called, "the old man of the woods" dancing around a burning log; a log that came to be known as the Yule log. The old man was characterized as having a red-nose, a white beard, and being a jolly old boozer who danced around the fire proposing toasts.
When Roman missionaries began to Christianize the lands to their north that Teutonic festival blended with the celebration of Christmas. The old man of the woods got a new name and became "Kriss Kringle.
Enter Nicholas, a 4th century bishop, noted for his kindness and gifts. He became Saint Nicholas and blended with Kriss Kringle. Eventually Dutch settlers brought his legend to the new world, but dialects distorted the Dutch name Sint Nicholas to Sinterclaas that in turn became Santa Claus.
It was a Washington Irving story in 1809 that probably helped the legend develop even further. He had a jolly Saint Nicholas, smoking a pipe, flying through the air in a wagon dropping presents down the chimneys.
Several years later, in 1822, Clement C. Moore, wrote the poem "The Night Before Christmas". The wagon became a sleigh and reindeer pulled it through the sky.