Answer:
vernal equinox
Explanation:
So, in the Northern Hemisphere you have: Vernal equinox(about March 21): day and night of equal length, marking the start of spring. Summer solstice (June 20 or 21): longest day of the year, marking the start of summer. Autumnal equinox(about September 23): day and night of equal length, marking the start of autumn.
