Leap years add an extra day every four years to keep the calendar year synchronized with the astronomical year. Seasons and astronomical events do not occur in whole number days, so a calendar with 365 days each year would drift over time. The Gregorian calendar, used worldwide, makes years divisible by 4 leap years, except for years divisible by 100 which are not leap years unless also divisible by 400. This makes the average year 365.2425 days to closely match the solar year.

1. Logic behind Leap Year
A leap year (or intercalary year) is a year containing one extra day (or, in the case
of lunisolar calendars, a month) in order to keep the calendar year synchronized with
the astronomical or seasonal year.
For example, in the Gregorian calendar (common calendar), February in a leap
year has 29 days instead of the usual 28, so the year lasts 366 days instead of the
usual 365. Because seasons and astronomical events do not repeat in a whole
number of days, a calendar that had the same number of days in each year would,
over time, drift with respect to the event it was supposed to track. By occasionally
inserting (or intercalating) an additional day or month into the year, the drift can be
corrected. A year that is not a leap year is called a common year.
Gregorian calendar
In the Gregorian calendar, the current standard calendar in most of the world, most
years that are evenly divisible by 4 are leap years. In each leap year, the month of
February has 29 days instead of 28. Adding an extra day to the calendar every four
years compensates for the fact that a period of 365 days is shorter than a solar year
by almost 6 hours.
However, some exceptions to this rule are required since the duration of a solar year
is slightly less than 365.25 days. Years that are evenly divisible by 100 are not leap
years, unless they are also evenly divisible by 400, in which case they are leap
years.
For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not.
Similarly, 2100, 2200, 2300, 2500, 2600, 2700, 2900 and 3000 will not be leap years,
but 2400 and 2800 will be.
By this rule, the average number of days per year will be
365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes & 12
seconds. The Gregorian calendar was designed to keep the vernal equinox on or
close to March 21, so that the date of Easter (celebrated on the Sunday after the
14th day of the Moon—i.e. a full moon—that falls on or after March 21) remains
correct with respect to the vernal equinox. The vernal equinox year is about
365.242374 days long (and increasing).
For more details: - follow this link
http://en.wikipedia.org/wiki/Leap_year
Dhananjay Kumar
PGP-09-11-017