Overview
September 1 is the 244th day of the year in a common (non-leap) year and the 245th day in a leap year. In either case there are 121 days remaining until the end of the year. The date sits near the astronomical autumnal developments in the Northern Hemisphere and is the first day of meteorological autumn there, while it marks the start of meteorological spring in the Southern Hemisphere.
Position in the calendar
Because September 1 follows 243 full days in a common year, it is the opening day of the final third of the year. The relationship between day counts and fractions of the year changes when a leap day is present, but the date retains the same ordinal placement relative to month numbering. The numbering and seasonal meaning of September 1 depend on the calendar system and local clock rules such as daylight saving time.
Name and historical origin
The name "September" derives from the Latin septem, meaning "seven." In the early Roman calendar it was the seventh month; after calendar reforms that added January and February it became the ninth month but preserved its original name. Modern use of September 1 follows the Gregorian calendar system, which corrected inaccuracies in the earlier Julian calendar; for background see the Gregorian calendar.
Time-of-year fractions and clock effects
The precise instant that marks the two-thirds or final-third point of a year falls at a specific hour of a specific day and therefore varies with time zone and with observance of daylight saving (summer) time. In a common year, the two-thirds point occurs after 243 days and 8 hours have elapsed (leaving 121 days and 16 hours), so local time offsets can shift whether that instant falls on September 1 or another nearby clock time. For details on leap-year adjustments and related calculations, see leap year explanations. These timing subtleties are why commentators sometimes note different local times for the same fractional-year milestone; local civil clock rules determine the displayed hour.
Observances and cultural importance
September 1 is associated with several recurring observances in different countries and traditions. In many places it coincides with the return to schools and universities and is widely thought of as the start of the academic year. Several Eastern Orthodox Churches observe the beginning of the liturgical year on September 1. In several post-Soviet states the date is celebrated as "Knowledge Day," marked by ceremonies for students and teachers.
Notable events and references
Historically, September 1 is remembered for major events that have fallen on the date; one of the most widely known is the German invasion of Poland on September 1, 1939, which precipitated the wider conflict of World War II. The date also appears in literature and music: for example, it is referenced in works that respond to or reflect on 20th-century history. For general timelines and lists of specific events by date consult chronologies or almanacs such as those linked at reference resources.
- Day number: 244 (common year), 245 (leap year).
- Days remaining in year: 121.
- Named from Latin "septem" (seven) despite being the ninth month today.
- Associated observances: academic-year openings, Knowledge Day, start of liturgical year (Eastern Orthodox).