The seasons officially change when the day and night are equal in length. In the Northern Hemisphere, sprint begins.
An equinox happens twice a year, when day and night are essentially the same length of time. It occurs when the earth’s equator is on the same plane with the center of the sun and the planet’s axis is pointed neither toward nor away from the sun.
The equinox is technically a specific moment in time, although it is common for the term to be used to refer to the day in which the equinox falls. In 2012, the March equinox occurs at approximately 11:02 UTC (5:02am EST) on the 20th.
In the United States, the March equinox is often referred to as the vernal equinox since it is the start of spring. In the southern hemisphere, however, the March equinox signals the beginning of autumn.