From this short article on the evolution of Santa Klaus, we see that Santa was first associated with Christmas Eve in 1821. That is not that long ago, historically speaking. Here's more on the origins of Christmas, and look around for more on the Christmas Tree, the reindeer and the 'Twas the Night Before Christmas'.
This pastor says: "Santa Claus, in and of itself, can be a fun custom. Just don't call it Christmas!" St.Nicholas day is on December 6th. Read when and how it got moved to the 25th. Look around the site - the pastor has written several articles about Christmas, Santa Claus and the origin of the Christmas Tree. This short article is a rare one that implicates Coca-Cola advertising office for the popularity of the fat-old-red version of Santa.
Here is an illustrated guide to Santa Clauses of various eras and geographies and this is another short history of Santa in America.
Here, another pastor did research on the evolution of Santa, more pictures here, and you can read even more in book form.
For me, reading this was always fun - an engineering professor explaining Santa's allnighter feats. And here is how to track him.
There are many copies of "Physics of Santa Claus" on the Internet. Here's one copy. This one appears mutated. And this one, in full spirit of scientific investigation, begs to disagree.
Hey, I grew up loving New Year's Eve. It is to this day my favourite holiday. Why? Because where I come from, on New Years' Eve you have a party, you decorate the New Year's Tree, you eat great food, and Grandfather Frost (Deda Mraz) puts presents under the New Years' Tree. Of course, he arrived in a sleigh pulled by reindeer, and got down the chimneys although he is fat and old and wears a funny red suit. We all sent each other New Year's Greetings cards and told "Happy New Year" to each other well into January! So, who said that Santa has anything to do with Christmas?