Ancient Greece...

It one of the ancient Greek traditions to held a Olympics every 4 years where people come from different places and come to challenge to see who the best at every different kind of sport
etc.

swimming,running,boxing...and so on.
It happen every 4 years because for some challenger it take they about 6 months to go back to their hometown and they also held it at different places to make it easier for some of the challenger to get there.
In the Olympics you will get different medal that only the country have (medal=a crown make of leaf that can only be found at that places)and when you win you will get a hero's welcome when you get back to your hometown.