Even though we call 365 days a year, the earth does not revolve around the sun in 365 days. Rather, it takes 365 days, 5 hours, 48 minutes, and 46 seconds to do this.

The extra time is made up by adding one extra day to the end of February every four years, EXCEPT in those years which can be divided evenly by 100. Then that extra day is NOT added. However, in years divisible by 400, that extra day IS added.

What that means is simply this, the years 1200, 1600, and 2000 are divisible by 400, so they have the extra day added, making them Leap Years. However, 1300, 1400, 1500, 1700, 1800, and 1900 are not divisible by 400, so they were not Leap Years.

Every year divisible by 4 is a leap year (adds an extra day to February),

EXCEPT the last year of each century, such as 1900, which is NOT a leap year . . .

EXCEPT when the number of the century is a multiple of 4, such as 2000, which IS a leap year . . .

EXCEPT the year 4000 and its later multiples (8000, 12000, etc) which are NOT leap years.

my birthday is on leap year

A leap year (or intercalary or bissextile year) is a year containing one additional day (or, in the case of lunisolar calendars, a month) in order to keep the calendar year synchronized with the astronomical or seasonal year.

we have leap year because the earth did not rotate around the sun exactly 365 day and there is one day added on the calendar every 4 year.