Did you think, as I did, that the equinoxes and solstices were all were on the 21st (of March, September, June, and December, respectively)? We went over why the spring equinox was on the 20th this year in this post here, by the logic of which, the autumnal one would be early rather than late, yes? Not so, evidently! Furthermore, it seems that an autumnal equinox on the 21st is quite rare! Read all about it at this site here (scroll down to #5).