Gregorian calendar, New Style calendar - the solar calendar now in general use, introduced by Gregory XIII in 1582 to correct an error in the Julian calendar by suppressing 10 days, making Oct 5 be called Oct 15, and providing that only centenary years divisible by 400 should be leap years; it was adopted by Great Britain and the American colonies in 1752

Michaelmas, Michaelmas Day, September 29 - honoring the archangel Michael; a quarter day in England, Wales, and Ireland

Labor Day - first Monday in September in U.S. and Canada

Citizenship Day, September 17 - celebrated in the United States

American Indian Day - US: the 4th Friday in September

Gregorian calendar month - a month in the Gregorian calendar

mid-September - the middle part of September

autumnal equinox, fall equinox, September equinox - September 22

autumn, fall - the season when the leaves fall from the trees; "in the fall of 1973"