Leap years almost always occur every four years, (i.e. in years that are divisible by 4). The only exceptions to this is that years that are evenly divisible by 100 are not leap years, unless they are also divisible by 400.

So this means that the years 1700, 1800, 1900, etc., were not leap years, but the year 2000 was, and the years 2400 and 2800 will be.

en.wikipedia.org/wiki/Leap_year

A leap year is a calendar year containing one additional day added to keep the
calendar year ... By this rule, the average number of days per year is 365 + <sup>1</sup>⁄4 −
<...

www.wwu.edu/skywise/leapyear.html

There is a leap year every year whose number is perfectly divisible by four -
except for years which are both divisible by 100 and not divisible by 400.

www.timeanddate.com/date/leapyear.html

Feb 29, 2016 ... A leap year has 366 days, as opposed to a common year, which has 365. ... one
rule: any year evenly divisible by four would be a leap year.

www.infoplease.com/spot/leapyear1.html

This new rule, whereby a century year is a leap year only if divisible by 400, is the
sole feature that distinguishes the Gregorian calendar from the Julian calendar ...

www.infoplease.com/spot/leapyear2.html

Rules for Determining a Leap Year. 1. Most years that can be divided evenly by 4
are leap years. (For example, 2016 divided by 4 = 504: Leap year!) 2.

support.microsoft.com/en-us/kb/214019

Dec 17, 2015 ... Any year that is evenly divisible by 4 is a leap year: for example, 1988, ... only the
year 1900 is subject to the 100/400 exclusion rule of leap ...

www.huffingtonpost.com/jacob-stewart/think-leap-year-is-every-_b_9131516.html

Feb 29, 2016 ... Because for the most part, Leap Year is every four years. ... Well according to the
rules of the Gregorian calendar, the exceptions to this rule are ...

www.telegraph.co.uk/news/uknews/12177017/Leap-Year-2016-Why-does-February-have-29-days-every-four-years.html

Mar 1, 2016 ... Leap Year 2016: Why does February have 29 days every four years? .... The
added rule about centuries (versus just every four years) was an ...

www.theguardian.com/science/2011/feb/28/leap-year-alex-bellos

Feb 28, 2011 ... Leap years are divisible by four of course – but it's a little more complicated ...
The leap year rule that has been in place since 1582 is a bit more ...