Today at 11:44 UTC (roughly the same as GMT).
The center of the Sun crosses the equatorial plane from south to north. On the equator the sun passes directly overhead. At the poles the Sun is on the horizon the whole day. On this day the Sun spends roughly the same length of time over and under the horizon so day and night are roughly equally long at all places on Earth.
Vernal equinox comes from the latin ver = spring, aequus = equal and nox = night (day and night of equal length).