| | Start Date | End Date | Best For... | | :--- | :--- | :--- | :--- | | Astronomical | Sept. 22-23 (varies) | Dec. 21 | Tradition, skywatching, solstice celebrations | | Meteorological | September 1 | November 30 | Weather records, climate statistics, heating bills | | Phenological | First leaf color / first frost | First permanent snow | Gardening, hunting, nature observation | | Cultural (US) | Labor Day (early Sept.) | Thanksgiving (late Nov.) | Retail, sports seasons, school calendars | The Verdict If you are an astronomer or a pagan celebrating Mabon, autumn starts precisely at the equinox. If you are a climatologist or paying your heating bill, it starts on September 1. But if you step outside, take a deep breath of crisp air, and notice the slant of the golden afternoon light, you’ll know the real answer: Autumn begins the moment you first feel it in the air. For most of the temperate world, that moment arrives sometime in mid-to-late September, though its official entry on the calendar will always be a subject of friendly debate.