Equinox

Equinox
noun

Definition: a day in which the sun passes the plane of the equator, making night and day exactly equal in length
Synonyms: vernal equinox, autumnal equinox
Antonyms: solstice

Sample Sentences:

There is a myth that an egg can balance on its end only during an equinox, when time is also balanced between day and night.

Sally chose the vernal equinox as the first day of her spring cleaning.

Usage:

http://www.livescience.com/23445-fall-equinox-seasons-change-explained.html

http://www.space.com/17697-autumnal-equinox-start-fall.html

Paragraph:

The two equinoxes each year are March 21 and September 23. These dates also mark the end of winter/beginning of spring and end of summer/beginning of fall.

Visual Representations: