The equinox - derived from the Latin aequus (equal) and nox (night) - is the day when daytime and night are of approximately equal duration which occurs twice a year, around 20 March and 22 September.
When Julius Caesar established his calendar in 45 BC, he set 25 March as the spring or vernal - from Latin ver for spring - equinox. Of course, he didn't know about us down here where the spring equinox occurs in September which is why the names March equinox and September equinox are gaining popularity since they are without the ambiguity as to in which hemisphere they occur.
Well, there's nothing ambiguous about the September equinox to me: it's that day of the year when I gradually swap insomnia for indolence as the sleepless nights shorten and the lazy days lengthen.
