The date February 29 is known as leap day and the year containing leap day is known as leap year. According to Gregorian calendar the most of the years that are divisible by 4 is known as leap year such as 2000. 2004, 2008, 2012, 2016 etc… are known as leap year.
1900 and 2000 are divisible by 4 but 1900 is not a leap year and 2000 is a leap year. That is because the years that are divisible by 100 and not divisible by 400 cannot contain a leap day. 1900 cannot be divisible by 400 but 2000 do.
In Gregorian calendar February 29th is the 60th day of calendar with 306 days remaining to complete the year. Many modern calendars have only 365 days and when a leap day comes it becomes 366 days.
To complete a revolution around the sun it takes 365 days and 6 hours. The extra 6 hours in each year is added in every 4 years and there arise requirement of a single day. So in each 4 years the extra hours are added up and a new day called leap year comes.2004 is a leap year.
Without adding this day the seasons will move back in the calendar and it will be confusing to take care the activities depending on whether, sunlight and ecology. A solar year is slightly shorter than 265 days and 6 hours. It is actually 365 days, 5 hours, 49 minutes and 16 seconds. So adding an additional day in every 4 year will add an additional 43 minutes and 12 seconds. So in each 400 years it will add up 3 days more. To compensate this the leap day after each 400 year is avoided. So a year will be leap year not only divisible by 100 but also divisible by 400 also.