September (Roman month) 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 to its position on the calendar. A day was added to September in the mid-40s
Adoption of the Gregorian calendar old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During and for some time after the change between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them. The Gregorian calendar was decreed in 1582 by the papal bull "Inter gravissimas" by Pope Gregory XIII, to correct a divergence in the canonical date of the [northern] spring equinox from observed reality (due to an error in the Julian system) that affected the calculation