The equinox is different to the equilux. The term for when the days and nights are equal length is the equilux:
The Equinox and Solstice
WWW.METOFFICE.GOV.UK
Equinox and solstice are vital parts of the astronomical calendar which determine the transitions between the seasons.
The equinox very rarely occurs on the 21st Sept as it happens. It's a date that has somehow got drilled into our mindset but it's extremely rare and is only occurring twice this century.
Nick L's post in Equinox Conundrum was marked as the answer