This year, I saw an awful lot of people get upset, because the Catholic Church is responsible for Christmas being on December 25. It is true, Pope Saint Julian I made this the official date for Christmas, in the year 350. Why did he do this?
1. One reason he did this was for the symbolism. December 25 is around the winter solstice, when the days start getting longer and there is more daylight. That is a good metaphor for Jesus Christ, the light of the world, who was born into a world made dark by sin and evil, so he could bring light to fallen humanity. "This life was the light of the human race; the light shines in the darkness, and the darkness has not overcome it." (John 1:4-5)
The downside is that pretty much every religion has an event for the winter solstice, so Christianity gets accused of copy-catting the religions that came before it. Because winter is dark, most religious events that take place in winter involve light, such as Jewish Hannukah and the German bonfires and Christian celebration of St. Lucy.
2. With this and other calendar work that he did, Pope Saint Julian did not pick dates out of a hat at random. He polled the Christians who were living at the time, as to when they celebrated the birth of Jesus. December 25th was one of the common answers; dates in February and September were also common.
I guess if people in the 300's didn't know the exact date for sure, us people 1700 years later don't have a chance of getting it right.
3. Another reason for picking December 25th for Christmas is that it is exactly one week before New Year's Day. In accordance with Jewish law, Jesus was circumcised a week after being born. So by assigning the Feast of Christmas on the 25th, that automatically assigns a feast on New Year's Day. I have never heard of anyone who complains about having New Year's as a holiday.
Also, the Feast of the Epiphany (celebrating the presentation of Jesus to the Gentiles / when the wise men visited the Christ child) was already set on January 8th. December 25th is two weeks before that, so it was a good fit.