We know that leap year occurs after every four years. Earth revolves around the sun in 365 days, five hours, 48 minutes and 46 seconds but we take one year of 365 days. There is still five hours left for the earth to complete its revolution around earth which is approximately Â¼ of a day. So to compensate with this difference experts had added a day in the month of February after every four years. If this would not be done then seasons will not match with the months and after 700 years winter will be in July and summer in January. To avoid all this we add an extra day in February. So leap year has 366 days.
Earlier Egyptians used solar calendar in which they take a year of 365 days. Then in 1582 first day of spring came on March 21st. Church leaders were worried that Easter might fall in winters. Then in 1583 Pope Gregory XIII ordered to shorten that year by ten days and started a new practice of adding a day after every four years.
We have noticed that earth takes 365.242199 days to complete one revolution which is less than 365.25 days. So adding one day after every four years will increase the time. To further compensate with it we do not add a day in the years which are divisible by hundred and not by 400. This happens thrice in four hundred years and these years are not counted as leap years. Year 2000 is a leap year because it is divisible by 100 as well as 400. 1700, 1800, 1900 were not leap years as they are not divisible by 400.
It is named as leap year because every year we have 52 weeks and one day, it pushes the day forward in next year week for e.g. if there is Sunday on 1st march next year it will be Tuesday and then Wednesday. Leap year causes one more day further, so because of this it is named as leap year.