We know that Christmas is to celebrate the birth of Jesus and that is was adopted from the Pagans to do so. Many biblical scholars agree that Jesus was actually born during the Spring, possibly around April. There are some who say the three wise men did not appear at Jesus' doorstep until he about two years old (so much for the traditional nativity scene).
My guess it that the Christians simply adopted the Winter solstace so that they would have some dominence over both times of the year and that the Pagans would be displaced from their old beliefs.
In a biblical sense, Christmas, taken as a celebration of the birth of Jesus as a savior, seems equally important as Easter is taken as that savior resurected. All that is, of course, based on faith in the teachings of one's church of choice.
My take is that Easter is, or should be, the more solemn of the two even though Christians rejoice at the resurection part. Christmas, while deep in religious tradition, is more of a joyful celebration and gift giving seems to be part of biblical history. From there, the entire concept has been commercialized to pay homage to a host of manufacturers, wholesalers, and retailers. They are getting a decent grip on Easter, too.
Most of you know me as an atheist, but I do try to learn about such things. I haven't found anything yet that will change my way of thinking, but I do find the commercialism of Christmas a travesty to the fundamental beliefs of Christians.