It’s on Friday or Saturday, depending on where you live.
Technically, autumn arrives in the Northern Hemisphere on Saturday, September 23, at 4:03 a.m. Universal Time (UT). That’s 5:03 in the morning British Summertime; three minutes after midnight U.S. Eastern Daylight Time; 11:03 p.m. on September 22 U.S. Central Daylight Time (=blog time); and 9:03 p.m. on September 22 on the American West Coast.
You’ll have to make adjustments if you’re in Arizona, which doesn’t observe Daylight Saving Time. And, of course, for New Zealand and the rest of the Southern Hemisphere, this is the spring equinox. Whew.