The best person to answer your question would be
Marilyn Agee, she addressed this question a couple of dozen times on
her website. I'll give it a stab. The Jews have two annual calenders a
sacred, which is commanded in Leviticus and a regenal or belonging to
the King, or government. While the priests kept the sacred, the late
era of the Kings needed another to run the government. There's also the
issue of 'lunar' calenders and calenders based upon the Sun. We live on
a solar calender, which tells us when to plant crops etc, The Jews delt
with two calenders.
Look how anal the Jews were about Levitical law 614
laws that nobody could keep. Imagine trying to keep the holy days in
ancient Israel while obeying two calenders. Today there's a compromise,
which takes some detail from both.