Every Four Years
In the United States, some special happens every four years:
Every four years, is a leap year and there are 366 days. February 29 as an intercalary day.
Leap years are needed to keep our modern day calendar year with the solar year, or the length of time that it takes the Earth to complete its orbit around the sun. This takes around 3650xBC days (officially 365.242).
This means that if we didn't add an extra day every four years, we would lose around six hours from our calendar each year, and every 100 years, our calendar would be out of line by 24 days.
Leap year started with the Gregorian calendar which started on October 15, 1582.
In 1986, the International Olympic Committee changed the schedule and now an Olympic event happens every 2 years. The first Olympic to encounter the change was the 1992 Winter Olympics, which had a 2 year duration between the games. This changed allows athletes to participate in both Olympics. (Wouldn't you want to see Usain Bolt in the speed skating competition?)
Since 1789 the United States has had a Presidential Election every four years