It is the precise time at which the apparent longitude of the Sun is zero degrees. The actual date varies very slightly from year to year. In the ecclesiastical system the vernal equinox is fixed at March 21 regardless of the actual motion of the Sun. The date of Easter is a specific calendar date. Easter starts when that date starts for your time zone.
Astronomical phenomena occurs at a specific date and time all over the Earth at once.
Inevitably, the date of Easter occasionally differs from a date that uses an astronomical full moon and the astronomical vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth. 1962 was such a case. In 1962, the astronomical full moon happened to occur about six hours after astronomical equinox. The ecclesiastical full moon, however, occured on March 20.