Why does February only have twenty-eight days?

Answer 1

Not always, sometimes it has 29 days.

The most likely explanation that I can think of goes back to the Romans when the year began in March. The months alternated between 31 and 30 days The Romans knew there were roughly 365 days in the solar year and so after 11 months there would only be 28 days left for the last month. The months were named after the numbers, hence September was month Seven or Septem. Similarly October was month Eight or Octo, etc.

July was originally month Five, but was renamed in honour of Julius (Caesar). Similarly August was originally month Six, but was renamed in honour of Augustus (Caesar). As they didn't like Julius' month having more days than Augustus' month, the number of days were switched for months 6-11 so that there are two months with 31 days together; September then had 30 days instead of 31, October 31 instead of 30, etc.

January became the beginning of the year with the Gregorian reform which corrected for the extra leap days inserted as the Romans thought the year was exactly 365.25 days long - it is actually slightly shorter than this - the current system of a century being a leap year if, and only if, it is evenly divisible by 400 will need correction, but not for a few millennia hence.

The French had a "metric" calendar which had 12 months of 30 days each of 3 weeks of 10 days each; this left 5 or 6 (in a leap year) extra days which were a national holiday at the end of the year. It lasted about 12 years before being scrapped.