What is Meant by Indiction?

The word “indiction” comes from the Latin word “indictio”, which literally means “institution, proclamation, appeal, announcement.” The “indiction” was an edict (or order) of the Roman Emperors used to determine the land tax through the Roman Empire. Such edits began to appear during the reign of Diocletian (284-305) in the year 297 A.D. At first, they were issued every five years, then later every fifteen years. Gradually the word “Indiction” came to denote not only an imperial proclamation, but also a fifteen year cycle as well as the first day of this cycle. Originally, the indiction was used exclusively for fiscal and tax purposes. But slowly it began to be used in determining the various dates of civil life. This fiscal year did not coincide with the astronomical year which, since the reform of Julius Caesar in the year 46 to the coming of Christ, began with the first day of January. The first day of the indiction was originally the twenty-third of September because that was the day on which Caesar August was born, but under Constantine the Great (306-337) it was the first day of September.
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The Beginning of the Indiction—The New Liturgical Year

The Fathers of the First Ecumenical Council in Nicea in the year 325 adopted the first of September as the opening of the New Church Year and this day has been observed in the Eastern Church to the present time. The Latin Church opens its Liturgical Year on the first day of Advent, i.e., the beginning of the preparation for Christmas.

The indiction of which we are speaking—for there were other indictions—is called the Byzantine (or Constantinopolitan) indiction which, except for Egypt, became mandatory throughout the Roman Empire. Justinian I (527-565) made dating by indiction compulsory for all legal documents. The Roman church during the reign of Pope Pelagius II (579-590) adopted the indiction for establishing the dates of documents, and this practice was not abandoned until the year 1097.