Why does February have 28 days?
The Romans borrowed the first calendar long ago from the Greeks. The Greeks had a year of 10 months, consisting of 304 days. The remaining days that fell in winter were not bothered about, as the calendar was supposed to support farming activities. So, the annual calendar ran from March till December.
About 2700 years ago the Roman king, Numa Pompilius, added January and February to the calendar. Even then the calendar had only 355 days. This is because each of the months were taken as a lunar cycle. The 12 months had the days that spanned a lunar cycle, and hence, it comes to around 355 days. A new month had to be added to put the back the year on track. So king Numa added a month called Mercedinus.
February is the second month in the civil calendar of the Romans, while it is the last month in the religious calendar of Rome.