The history of the calendar is pretty tangled. It dates back to Roman times. The Roman calendar originally had only 10 months, beginning on March 1. (That's why "September" is named after "7", "October" after 8, "November" after 9, and "December" after 10.) After December, there was just a long wasteland that nobody really cared about. In the earliest calendars, these were probably 29 day lunar months, but that got stretched out to 30 and 31 days over time. This was jiggered further around 713 BCE to add January and February. February was supposedly handed 28 days so that it would be the only month with an even number of days, while the others all had 29 or 31. Even numbers were considered unlucky, but somebody had to have one*; might as well make it the short and unpleasant month. The calendar was fiddled with still further under Julius Caesar, who eliminated the leap months (like with the Jewish calendar, they still needed leap months to keep it aligned with the solar year) and made February a leap year more often. That largely settled the calendar, though they continued to make changes for the next 1500 years or so to give us the calendar we have today. Nobody ever wanted to make February any longer so it remains at 28 days.