Why Does Only February Have 28 days?

Have you ever wondered why only the month of February has 28 days{or 29 in case of a leap year}? 

Mostly because of maths and the moon. This started in the eighth century B.C. when the Roman calendar was figured out. Until then there were only ten months- no January or February at all.

Numa Pompilius, a Roan king, who decided to make a change and add 2 months. He worked the year out to 355 days(12 lunar cycles). The Romans were superstitious about even numbers, though, believing that these numbers were bad luck, so the Roman calender had only  29 or 31 days months. To arrive at 355 days there had to be one shorter month, and February was chosen. March was the beginning of the year then, so February was the last month in the dead of winter. Maybe the Romans also figured that if one month had to have some bad luck attached to it because of the even number of days, it might as well be a short month.
Julius Caesar gave the calendar another once much later, and bumped the total up to 365 days. He kept February short because it was a month that no one really liked. His calender is called Julian calendar, and was used until 1582. Then Pope Gregory XIII worked out his version, the Gregorian calender, which is the one we use today.
A calender year is exactly 365 days, 5 hours, 48 minutes, 46 seconds, so an extra day was added every fourth year to account for those extra hours. That's when we get a leap year, when the year is exactly divisible by 4. The date February 29 is added to make up the total number of days that are required over 4 years.