Why Does Our Tefillah Change on December 4th (or 5th)?

Rabbi Ethan Tucker

Birkat HaShanim

The ninth blessing of the Amidah, called Birkat Ha-Shanim, is variable depending on the time of year. In Diaspora, this change happens on either December 4th or 5th. How did this happen? How did our tefillah end up depending on secular dates?