Final answer:
On March 21, all places on Earth experience the same period of day and night due to the vernal equinox.
Step-by-step explanation:
On March 21, all places on Earth experience the same period of day and night. This is because March 21 is the date of the vernal equinox, which marks the beginning of spring in the Northern Hemisphere. During the vernal equinox, the Sun is directly above the Earth's equator, resulting in equal lengths of day and night for all locations on Earth.