When the Sun is directly above the Equator at noon(?). Same as the Vernal Equinox in Spring.
And so therefore, the hours of daylight equal the hours of nighttime.
Unless you're on Daylight Savings Time.
-PJ
The Spring and Fall equinoxes actually do bring in their name sake weather, whereas the Summer soltice and the Winter soltice do not — the name sake weather has been in place for some time. I notice that in Europe, the Summer soltice is called midsummer.
I see these idiot weather people on TV look into the camera and talk about the hot/weather and proclaim, “It is hot/cold and Summer/Winter is not even here yet.”