as we know today came from 19th century American cartoonist. The old man riding a sleigh with reindeers pulling it across the sky was perhaps borrowed from Odin's myth. Where Odin rode a eight legged horse across the north Sky to hunt during Yule. However, Odin didn't give gifts. That reindeer thing was from an old poem, I think.

Again, long before Santa Claus, Christmas already exists, as you know it, the Christian missionaries christianized Germanic Yule. Making the day to be the birth of Christ was basically trying to counter the pagan festival. That however, is not directly related to Santa Claus. The dutch Sinterklaas day was actually Dec. 6.