During which phenomenon would the lengths of day and night be equal?

The phenomenon during which the lengths of day and night are equal is the Vernal Equinox. This occurs twice a year, in March and September, when the Earth's equator passes directly under the sun. During the equinox, the tilt of the Earth's axis is such that sunlight is distributed evenly across the entire planet, resulting in almost equal amounts of daylight and darkness at all latitudes.

This event signifies the transition into spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is a fundamental aspect of the solar calendar and is significant in various cultures as it marks a time of balance and changing seasons.

In contrast, the Summer Solstice and Winter Solstice are specific points in the year where the lengths of day and night are not equal, with the Summer Solstice having the longest day of the year and the Winter Solstice having the shortest. The term "Summer Equinox" is incorrect, as equinoxes refer specifically to the periods when day and night are equal.