The Nativity : Chapter 12-97

In other words, the seven days became longer than the usual winter solstice. Therefore, on December 25 became the winter solstice.
(The date shifted seven days on calendar).
As the Roman calendar had the irregular dates, Holy Mother Mary, Jesus Christ, and the Jews did not adopt the Roman calendar. Before 8 A.D., the Jews adopted the old solar calendar of the Jews and the old Jewish calendar.