Isthmian Games
The Olympiad was one of the ways the Greeks measured time. The Olympic Games were used as a starting point, year one of the cycle; the Nemean and Isthmian Games were both held (in different months) in year two, followed by the Pythian Games in year three, and then the Nemean and Isthmian Games again in year four. The cycle then repeated itself with the Olympic Games. They were structured this way so that individual athletes could participate in all of the games. (Note that the dial on the Antikythera mechanism seems to show that the Nemean and Isthmian Games did not occur in the same years.)