Because the leap year is happened after every 4 years.the 28 day is come in every common years and 29 days comes after the leap year.

Evry fourth yr we add an extra day to the month Feb as we have the least number of days in that particular month the earth takes 365 1/4 days to complete one revolution around its orbit hence we consider 365 days as a year and the 1/4 day is added after 4 year to make a day that is 1/4+1/4+1/4+1/4=4/4=1/1=1 and why it is added in the month of Feb there is a story behind it I don't knw about that in brief BT surely u can search on google