The Mass of Christ is on a specific, annoyingly incorrect, day.
(I don't like secular Christmas.)
Discussion
Annoyingly incorrect?
Jesus was born in the spring.
How do we know?
The Bible. The shepherds were out in the fields when they saw the heavenly host.
Springtime.
So I don't know enough about historical shepherding practices to speak to that, but here's what I do know:
In the early days of Christianity, there was a tradition that a person died on the same day of the year they were conceived. The date of Jesus' crucifixion and death was attributed to March 25th, in correspondence with the Jewish Passover in that year. Thus, early Christians figured that Christ was conceived on March 25th, which is still celebrated as the Feast of the Annunciation.
Go forward nine months, and it stands to reason that Jesus was born on December 25th. Thus, the date for Christmas was set to December 25th.