October (Roman month)

October (from Latin octo, "eight") or mensis October was the eighth of ten months on the oldest Roman calendar. It had 31 days. October followed September (from septem, "seven") and preceded November (novem, "nine"). After the calendar reform that resulted in a 12-month year, October became the tenth month, but retained its numerical name, as did the other months from September through December.

Some of the observances in October marked the close of the season for military campaigning and farming, which commenced in March (Martius, "Mars' month"). October was under the guardianship (tutela) of Mars.

The Romans did not number days of a month sequentially from the 1st through the last day. Instead, they counted back from the three fixed points of the month: the Nones (5th or 7th), the Ides (13th or 15th), and the Kalends (1st) of the following month. The Nones of October was the 7th, and the Ides was the 15th. The last day of October was the pridie Kalendas Novembris, "day before the Kalends of November". Roman counting was inclusive; October 9 was ante diem VII Idūs Octobris, "the 7th day before the Ides of October," usually abbreviated a.d. VII Id. Oct. (or with the a.d. omitted altogether); October 23 was X Kal. Nov., "the 10th day before the Kalends of November."

On the calendar of the Roman Republic and early Principate, each day was marked with a letter to denote its religiously lawful status. In March, these were:

By the late 2nd century AD, extant calendars no longer show days marked with these letters, probably in part as a result of calendar reforms undertaken by Marcus Aurelius. Days were also marked with nundinal letters in cycles of A B C D E F G H, to mark the "market week" (these are omitted in the table below).

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