First day of autumn

what the first day of the week

What the first day of the week is does not decide autumn. It only decides which column a calendar prints on the left. ISO 8601 weeks start on Monday. Many U.S. wall calendars start on Sunday. Jewish weeks are counted toward Saturday as Sabbath; liturgical weeks in many churches treat Sunday as the first day. Autumn still begins on a date, or at an instant, not on “the first column.” In 2026, 1 September is a Tuesday. 23 September is a Wednesday. Those weekdays would sit in different columns on a Sunday-start grid and a Monday-start grid, and they would still be the same dates.

If you are asking the week question because you want to know whether the first day of autumn is a weekend, look up the weekday of the date you chose, then stop. Do not slide autumn to the nearest Monday to make a tidy week. Seasons are not workweeks. A school that “starts fall term” on a Monday is using a term date, not the equinox.

Write the week-start rule next to the autumn rule so they cannot overwrite each other. Sunday-start is a print choice. Monday-start is an ISO choice. Equinox is an astronomy choice. 1 September is a meteorology choice.

Meteorological northern autumn: 1 September through 30 November. Astronomical northern autumn: from the September equinox to the December solstice. 2026 equinox instant: 23 September, 00:06 UTC. Week start: Sunday or Monday by convention, not by leaf colour.

Two autumns, one September

Meteorologists use whole months so that climate records compare cleanly: autumn is September, October, November in the Northern Hemisphere. That is why 1 September is already “the first day of autumn” on a weather desk, including Tuesday 1 September 2026. Astronomers use Earth’s orbit. At the September equinox the Sun crosses the equator heading south. Northern astronomical autumn begins at that instant; southern astronomical spring begins at the same instant.

Because the 2026 instant is 00:06 UTC on 23 September, clocks in the Americas are still on 22 September. In U.S. Eastern Daylight Time that is 20:06 on Tuesday 22 September. In U.S. Pacific Daylight Time it is 17:06 on the 22nd. In much of Europe it is already early Wednesday the 23rd. The first day of astronomical autumn is therefore Tuesday or Wednesday in 2026 depending on the zone. Say the zone. A headline that prints only “23 September” is using UTC’s date.

The December solstice ends astronomical autumn in the north and begins astronomical winter. Meteorological winter begins 1 December, which is earlier. That is why a weather summary can call December winter while a sky calendar still says autumn for the first three weeks of the month.

Leaves do not read UTC. Your calendar should. If the equinox happens at 00:06 on the 23rd in Greenwich, it is still the 22nd after supper in Chicago.

The other hemisphere keeps the opposite season

On the same equinox, the Southern Hemisphere enters astronomical spring. Their meteorological spring is 1 September through 30 November. So 1 September is already spring on a southern weather desk and autumn on a northern one. Christmas in December is then midsummer there. Do not send a northern “first day of autumn” packing list to a city where the trees are budding. The date travels. The season sign flips.

If you needed a countdown, count to 1 September for meteorological autumn in the north, or to the equinox instant for astronomical autumn. From 3 September 2026, meteorological northern autumn has already begun, two days earlier. The equinox is still ahead: 20 days to 23 September if you count UTC’s date and not the American evening of the 22nd.