Leap years occur every 4 years, except we skip one every 100 years, HOWEVER we skip that skip every 400 years.
2000 was the perfect example: /4 so leap year? well /100 so no leap year? well also /400 so no no = yes leap year.
Basically that one time in our lives we could have witnessed the /100 exception, it actually wasn't, so business (= leap year) as usual.