To respect the ancient origins of the Olympic Games, which were held every four years at Olympia.
The four-year interval between the Ancient Games editions was named an “Olympiad”, and was used for dating purposes at the time: time was counted in Olympiads rather than years.
In 1894, Pierre de Coubertin launched his plan to revive the Olympic Games, and in 1896 the first Games of the modern era were held in Athens. Today, an Olympiad begins on the first of January of the first year and ends on the thirty-first of December of the fourth year.