The first day of autumn in 2025 (astronomical/autumnal equinox) is Monday, September 22, 2025 in the Northern Hemisphere.

Quick Scoop: Different “first days”

  • Astronomical autumn (equinox): Starts Monday, September 22, 2025, when day and night are roughly equal and the sun crosses the celestial equator.
  • Meteorological autumn: Many weather services and calendars treat September 1, 2025 as the start of autumn for climate and record-keeping purposes.

So if you’re thinking in terms of the official seasonal change , use September 22, 2025; if you go by weather and monthly quarters, some people will say autumn starts September 1, 2025.

In forums and casual discussions, people often debate whether autumn “really” starts on September 1 or on the equinox, but official astronomical definitions point to the equinox date.

TL;DR:

  • Meteorological autumn 2025: September 1
  • Astronomical autumn 2025 (most common “first day of autumn”): Monday, September 22, 2025

Information gathered from public forums or data available on the internet and portrayed here.