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
Adoption of the Gregorian calendar the date of Easter astronomically using the instant of the vernal equinox and the full moon according to Kepler's "Rudolphine Tables" of 1627; this combination was referred to by the Protestant estates as the "improved calendar" ("Verbesserte Kalender") and considered to be distinct from the Gregorian. They finally adopted the Gregorian calculation of Easter in 1774. The remaining provinces of the Dutch Republic adopted the Gregorian calendar on 12 July 1700 (Gelderland), 12 December 1700 (Overijssel and Utrecht), 12 January 1701 (Friesland and Groningen) and 12 May 1701 (Drenthe). Sweden's transition to the Gregorian calendar was difficult and protracted. Sweden