The seasons officially change when the day and night are equal in length. In the Northern Hemisphere, autumn 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 2011, the September equinox occurs at approximately 9:05 UTC (5:05am EDT) on the 23rd.
In the United States, the September equinox is often referred to as the autumnal equinox since it is the start of autumn. In the southern hemisphere, however, the September equinox signals the beginning of spring.