It is believed that King Numa Pompilius decided that a year would have 355 days equivalent of the length of 12 lunar cycles. Similarly the each lunar cycle would be designated by a name. At that point of time even numbers were considered unlucky. So, he created seven months with 29 days, and four with 31. Now to account for 355 one even no. month was unavoidable with 28 days. So February was chosen as it was the least favorite month during the peak of winter. Hence, February has 28 days.