The designation of only 28 days in February began when King Numa Pompilius reformed the Roman calendar around 713 B.C. The Roman calendar then had only 10 months, with no months assigned to winter. Numa added January and February, with February as the last month of the year, and adjusted its length to correspond to a year of 355 days. The Julian and Gregorian calendar reforms later modified its length.
The word February comes from the Roman Februarius, which means purification. Whenever Roman calendars required modification, the changes were made after the 23rd day of February. The remaining days in February were considered a second part of the month. In the Roman calendar, leap months were added from time to time to reconcile calendar and solar years.
In 46 B.C., Julius Caesar introduced the Julian calendar. January became the first month of the year, and the year gained 10 days, increasing from 355 to 365 days. To realign the new calendar correctly, 46 B.C. was 445 days long, and afterwards, the years were of standard length. In the Julian calendar, a leap day was added to February every four years, making it 29 days instead of 28.
The next major reform was the Gregorian calendar, used in most modern western countries. The reform mainly concerns leap year calculations. It leaves out three leap days every 400 years to bring the calendar more closely in line with lunar cycles. It changed the average length of a year from 365.25 days to 365.2425 days, a difference of 10 minutes and 48 seconds every year. Through all these reforms, February remained the short month, as it was originally designated by Numa Pompilius.