the official start of either season is march 1, though the equinox can fall on march 20 or 21, occasionally on march 19. the northern spring equinox marks the start of the iranian new year and baha'i new year. it is from the march 21 date that easter's date is calculated, on the sunday after the first full moon in spring, meaning it can fall between march 22 and april 25 in western christianity.