vernal equinox (ˈvərnl ˈekwəˌnäks, ˈēkwə-): noun – the time or date* during the spring† at which the sun‡ crosses the celestial equator‡, when day and night are of equal length (about March 20).
* The equinox officially happened at 5:14am GMT.
† Spring in the northern hemisphere. It’s autumn down under.
‡ “Sun” and “equator” are euphemisms here. Ahem.