because earth takes 365 days and 6hours to complete its revolution
and 1st,2nd,3rd year the 6hours is not counted in these first 3 year
and on fourth year the 6 hours of first year ,second year,third year's 6 hours is counted on fourth year
As=====> 6hours + 6hours + 6hoursand fourth's year 6hours are also plus together=24hours
so 24hours = 1 day
so on 4th year365+1day =366days as become leap year.
so february becomes 29