It is because of Julius Ceasor and Numa Pompilius  .The moon takes 29 1/2 days to rotate around the Earth, while the Earth takes 365 1/4 days to rotate around the sun.before the calendar was lunar calendars and that time february was not there.Romulu had 10 months that added to 304 days.Later Numa Pompilius, Rome’s king after Romulus, tried to even out the difference by adding two new months to the calendar: January and February. Numa Pompilius’s version of the calendar now had 355 days, so he created a new month called “Mercedinus” that would come after February every other year in order to keep up with the solar rotation.In 45 BC, Rome’s leader Julius Caesar got rid of Mercedinus completely. Instead, he took those extra ten days and placed them at the end of the months they already had — some months now had 31 days, some had 30, and February, due to the loss of Mercedinus, would have 29. This system was called the “Julian calendar. Did you know when Julius Caesar decided to put the Julian calendar into practice, he had to wait for the perfect time for the solar rotation to match the calendar date — as a result, the year 46 BC was 455 days long :-).

In 1582, Pope Gregory XIII noticed that the Julian calendar didn’t take into account that 1/4 of a day included in the solar rotation. So he created the concept of the leap year in order to keep the solar calendar synchronized — February would now have 28 days a year, except in years divisible by four, in which they would have 29. These changes became the “Gregorian calendar,” which is the system we use to this day!.

Just to your reference

The early Roman calendar consisted of 12 months beginning in March like this (later January became the start of the year):

To know more you can check

http://askville.amazon.com/February-28-days/AnswerViewer.do?requestId=1329083

http://www.calendar-origins.com/roman-calendar.html

http://www.straightdope.com/columns/read/442/how-come-february-has-only-28-days