Page "Anno Mundi" Paragraph 17

: March 25 was considered to be the anniversary of Creation itself.

It was the first day of the year in the medieval Julian calendar and the nominal vernal equinox ( it had been the actual equinox at the time when the Julian calendar was originally designed ).

Considering that Christ was conceived at that date turned March 25 into the Feast of the Annunciation which had to be followed, nine months later, by the celebration of the birth of Christ, Christmas, on December 25.