Complex sentence: Nowruz is the day of the astronomical vernal equinox (or northward equinox), which marks the beginning of the spring in the northern hemisphere and usually occurs on March 21 or the previous/following day depending on where it is observed.
Split into simpler sentences: Nowruz is the day of the astronomical vernal equinox, and marks the beginning of spring in the Northern Hemisphere. It usually occurs on March 21 or the previous/following day, depending on where it is observed.