What was the event and what was the date from which later Greeks calculated their chronology (beginning) and from then on, despite their rivalries and differences, all Greeks considered themselves citizens of Hellas.

Event: First Olympiad

Date/Year: 776 BCE

Explanation:

The first Olympiad was conducted in 776BCE in honour of Zeus, the major sporting events are footrace, a javelin contest, and later wrestling matches were added.

Based on legend, the founder of the Olympic Games was Herakles (Hercules). Olympiad now known as Olympic in English language, is now a big events, in which all the countries in the world participated andit  it contains some interesting sporting events.