Why does the Orthodox calendar begin its fixed cycle of holy days on September 1?

The Orthodox Church begins its fixed cycle of holy days on September 1 because this date served as the New Year’s or Indiction Day of the Byzantine Empire.1

In the context of the Roman Empire, the indiction was a fiscal year system established under the emperor Diocletian to regulate annual tax levies. While the civil year began on January 1, the indictional year was reckoned from September 1. Because the Byzantine Empire maintained this administrative tradition, the church adopted the date as the starting point for the Menaion, the liturgical service book that organizes the invariable feasts of Christ, the Virgin Mary, and various saints throughout the year.21