There are a total of 365.26 days in one full calendar year. This number may vary slightly depending on the reference used. The number of days in a year has to do with the number of rotations that the Earth makes around the sun in one year. Since there is an extra one-quarter day (.26) every year, we have what is called Leap Year every fourth year. February normally has 28 days but during Leap year a 29th day is added to the calendar. This day is called leap day. This is based on the Gregorian calendar which the most accepted calendar in the entire world.

Actually the above answer is slightly off. There are 365.24 days. We make up one full day every fourt years but that means we go a tiny bit over. So there are no leap years on century years. However, even 365.24 is not exact and so every 400 years is a leap year. So the year 2000 actually had a leap day (2/29) but not 1900, 1800, or 1700.

A year may be defined in two different ways. We may define a year as a calendar year, or we may define it as the time taken by the earth to complete one revolution around the sun. This is called the solar, equinoctial or tropical year. A calendar year has 365 day in it, if it is not a leap year. A leap year has 366 days in it.

The time taken by earth to complete one revolution around the sun is about 365 and a quarter day. More precisely a year is 365 days, 5 hours, 48 minutes and 46 seconds long.

In calendar years we have the system of leap year so that we can have calender years in terms of complete days, and at the same time avoid discrepancy between duration of solar and calendar accumulating to a big number over a period.

The number of days in a year is 365 days.

Once in 4 years normally we observe a leap year.

But every 4th year is not leap. The century years are exceptional. Only the century years divisible by 400 is leap. Not other century years which are not divisible by 400.

So by this we can calculate that in 400 years there are 400*365days +400/4 days - 3days for 3 non leap century years = (146000+100-3)days = 146097 days.

So in one year there are 146097/400 days = 365.2425 on an average.

An year is the period earth takes to go round the sun and come back to to the exact /same position on the ecliptic. Normally in astrnomy we say this fixed point as 1st point of Aries. The first point of Aries also recedes by a very small measure called precission. So in astronomy this period of rotation is called a tropical year whose value is 365 days, 5 hours, 49 minutes, and 12 seconds nearly.