Originally, the Roman calendar had 12 months of 29 or 30 days, beginning in March, with an extra (intercalary) month thrown in every other year to get things back in sync with the seasons. Unfortunately this led to abuses, as officials added or dropped intercalary months to extend their time in office or to hasten elections.

So in about 45 BC, Julius Caesar, who by now had made himself dictator of Rome, established a 365-day calendar with leap years. For reasons unknown to me but which seemed good to him at the time, he decided that instead of having five months with 31 days and seven with thirty, he would only allow February to have 30 days in a leap year. He incidentally renamed the fifth month, Quintilis, after himself (Julius, or July).

Julius' adopted son and heir, Octavian, gained control of the state a few years after Julius' assassination in 44 BC. Octavian ruled so well that the Senate granted him the name of Augustus, and after his death he was formally deified by the Senate. As part of this, the month Sextilis following July was named August, just as Augustus had followed Julius as leader of the State. But since the month now named after the God Augustus had only 30 days, another day was taken from poor February and given to August.

It sounds silly, but that's the way it was.

My source is Encarta^TM.