Beginning in approximately 776BC, the Olympic Games played a vital role during the time of the ancient Greeks and became the symbol of spirit and unity in Antiquity. During this period, the Greeks were constantly at war. The only similarity that they acknowledged amongst each other was that they all spoke Greek. Apart from that, they seemed to think they were all very different from each other. This is the very reason why the Olympic Games were so important, it was the only period of time in which thousands of Greeks gathered together in harmony; there was no warfare. A treaty was formed by Ifitos, Lycourgos (the king of Sparta), and Cleisthenes (the king of Pissa) in order to terminate fighting in the whole of the Greek world during the time in which the games were taking place. .
The Olympics were held in Olympia every four years during the summer months of July or August and traditionally, they took place to honour the god Zeus. The time in between each Olympic Games was known as an Olympiad. The Greeks based their chronology on these four-year periods. The Olympic festival marked the beginning of each Olympiad. 776BC was counted as the 1st Olympiad.
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German Archaeological Institute- http://www.fhw.gr/olympics/ancient/classical/lezantes/29.html .
The surrounding area of the shrine of Zeus was given the name "Altis", which means grove. The Greeks honored Zeus and other gods at altars located in the Altis. From the sixth century and on, the Altis was decorated with temples, elaborate shrines, and statues. The Altis was the centre of all religious activities during the Olympic festival. A number of sacrifices were dedicated to the altars of various gods; the grandest of all being the sacrifice of one hundred oxen in front of Zeus" altar.
When the games were first established, they lasted only one day, and consisted of only a running event, but eventually, they extended into a five day occasion, not all of which were taken up by races however.