Before Christmas also, people used to celebrate winter season, but then after the birth of Jesus Christ it became more specific and formal. It is said that the traces of celebration can be tracked from 354 AD and they were celebrating Christmas on 25th December and then after certain years this tradition was imported to the eastern part who were earlier celebrating it on 6th January and during course of the time, things (Lot of) added to celebrations.
As per legends, in winter people used to celebrate (mostly European regions) and reason can be because there was were less agriculture activities during this time and people were waiting for the good season i.e. spring to come. But around 4th century A.D feasts were being celebrated in various places of earth, mostly the eastern parts. But as protestant rose around 15th century, they started to question the happening of Christmas.
Till now the most accepted thought are, what we hear and see and that is, Christmas is celebrated on 25th December every year. Yes there are several things that were added during the time which has passed. For example, Christmas tree and it all started when people started to decorate the tree with candles. So, it is sort of associated with the decorations and there is nothing wrong in that! Easter and thanksgiving are some of the other practices.
The main purpose of adding the same is that, they make people more socially available to each other, otherwise which would have been very difficult as per current work schedule. Santa Clause is a fictitious character and don’t have any origin in Christian religion, but got its inculcation during 17th century and has primary motive of giving gifts to the children.
More details can be taken from Bible, as this is the supreme source of knowledge for Christian religion. As per the saying goes, god Jesus wrote all his thoughts and messages of god in it and thus, it is the prime source of every work and deeds that a Christian should do in his or her life.