Unless you live in the Southern Hemisphere, in which case, Happy Vernal Equinox!
If you want to get technical, the equinox occurs when Earth’s rotational axis is perpendicular to the ecliptic with the sun. This will occur today at 0921 CDT, everywhere on Earth. Practically speaking, today there will be 12 hours from sunrise to sunset, and the days will begin to be shorter than the nights (again, in the Northern Hemisphere).
In other words, autumn has arrived!