September (Roman month) was adjusted to September 1 in the 5th century to coincide with the tax year. September (Roman month) September (from Latin "septem", "seven") or mensis September was originally the seventh of ten months on the ancient Roman calendar that began with March ("mensis Martius", "Mars' month"). It had 29 days. After the reforms that resulted in a 12-month year, September became the ninth month, but retained its name. September followed what was originally Sextilis, the "sixth" month, renamed "Augustus" in honor of the first Roman emperor, and preceded October, the "eighth" month that like September retained its numerical name contrary
December (Roman month) 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 December was the 5th, and the Ides the 13th. The last day of December was the "pridie Kalendas Ianuarias," "day before the Januarian Kalends". Roman counting was inclusive; December 9 was "ante diem V Idūs Decembrīs", "the 5th day before the Ides of December," usually abbreviated "a.d. V Id. Dec." (or with the "a.d." omitted altogether); December 24 was "IX Kal. Ian.", "the 9th day before the Kalends of Ianuarius," on the Julian calendar