Equinox is really 21st ……….. 21 March, 21 December, 21 June ………. 
The equinoxes are not these dates - they are just an approximation and the traditional days that the seasons start.
From what I've read the autumn equinox is never 21st even though this is the traditional start of autumn - and it is either 22nd 23rd or 24th - and to do with the fact that the year is longer than 365 days and other astronomical reasons. The spring equinox can occur on March 20th (as it will in 2016)!
This year it was yesterday September 23rd - although most people think of the start of autumn as 21st (notwithstanding the met office date for weather stats discussed earlier in Sept preferred by some
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Today has been
Hurdity
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