When does the equinox start?

The vernal equinox, and the autumnal equinox, happen in a single moment for the entire globe; they are not on-going phenomena that start at one time and end at a later time. The vernal equinox (in the north) will be March 20 2013 at 11:02 UT, which is for all practical purposes the same as 11:02AM, Greenwich Mean Time. This is the autumnal equinox in the Southern Hemisphere.

Each year the actual time of the equinox changes by several hours, a table showing all forthcoming times and dates can be found on the related link.