from ARRL

Objective: To work as many amateur stations in as many 2 degrees by 1 degrees grid squares as possible using authorized amateur frequencies above 222 MHz and all authorized modes of emission.

Date and Contest Period: First full weekend of August. Begins 1800 UTC Saturday, ends 1800 UTC Sunday (August 3-4, 2013). Entrants may use as much of this time as they wish.

http://www.arrl.org/august-uhf