My Discworld calendar has all sorts of holidays and holy days on it (most of them are real ones too!) but yesterday was apparently the Immaculate Conception.
Now follow this through: December 8th - conception, December 25th - birth. Now either that is also a damned miraculous birth as well or else Mary was pregnant for over a year or else someone in the church has been lying to us all this time and just randomly picking dates for holy days