22 March 2010
Well, even if the OED defines them as such, neither the September equinox nor the just-past March variety actually counts an exactly symmetrical twelve hours of daylight and darkness. Often, the fussy details of things in astronomy (like whether an equinox is labelled as spring, or just March) are related to an observers location on Earth. This time, though, it mainly matters that this was only the spring equinox if you live north of the equator, so identifying it by its month is less hemisphere-centric. Which is absolutely a word.