An Olympiad is a four year period, beginning with the games at Olympia.
1st Olympiad: 776-75 to 772-71 BCE
2nd Olympiad: 772-71 to 768-67 BCE and so on.
7th Olympiad: 752-51 to 748-47 BCE
75th Olympiad: 480-479 BCE to 476-75 BCE
This data comes from the Miller book listed in the sources.
Year 1 July - August (the full moon nearest the summer solstice) Olympia
Year 2 July - August Nemea
Year 3 April - May Isthmia
Year 3 July - August Delphi
Year 4 July - August Nemea
Year 4 April - May Isthmia
Year 4 (end) July - August Olympia (start of next olympiad)