The autumn equinox falls on September 23rd this year. Some years it is on the 22nd. Theoretically the day and night are of equal length that day, although I find it tends to vary a bit depending on where you live, like the solstices.
The equinoxes are also the time in the year when the length of the days and nights is changing fastest. Here in Denver that amounts to about two and a half minutes per day, but in northern Europe it's faster - almost four minutes per day in London, four and a half minutes in Edinburgh, close to five in Inverness, and over five in the Orkneys. I saw this when I lived in Juneau (5 minutes per day) where it really made an impression on me - the day/night cycle there is like a great tide of light, rising and falling with the seasons, all of life borne along on it.
Tomorrow I should have another reference book review. In the meantime, may you have a good autumn (or spring in the Southern Hemisphere) day.