Around 150 AD Catholic church leaders reinvented the Roman orgy, Saturnalia, the winter solstice, as the celebration of the birth of Jesus. Allowing for a bit of wiggle room, December 25th was the agreed upon date. With me so far?
Every good marketeer knows you won't enroll new members by gypping them out of something they always looked forward to. The Church was looking for new blood, so instead of saying, as they later would, "you cannot go to that stupid Roman party" they said, "Come to our new Christmas party". It worked.
Around the same time, a new feast day, Lady's Day was cut from whole cloth. Lady's Day marked the beginning of the liturgical year, and for all intents became the new New Years Day. Not uncoincidentally, Lady's Day, now known as Annunciation Day, was placed exactly nine months before Christmas, March 25. Today, if you will.
Have you ever wondered why the 8th month (oct ober) was really the 10th, or the 10th month (Dec ember) was 12th? Thank Lady's Day.
Around 1582, the Gregorian calendar was instituted throughout Catholic Europe. Its big improvement was to reinstate New Years on January 1, and moved the major liturgical celebrations to a more fixed time period, but didn't move the months.
And there boys and girls is your history lesson for the day. Why do I know this? It's my birthday.