Why do most months have 30 or 31 days and why does February have only 28 days?

Question

Why do most months have 30 or 31 days and why does February have only 28 days?

Answer 1

When July and August were added to the calendar, days were stolen from February because it was felt that a month named after a caesar should not be shorter than any other month.

Answer 2

The year is based on how long it takes for the earth to orbit the sun (originally, of course, it was based on how long it took the sun to precess along the ecliptic and return to its original position relative to the background stars, but this is the same thing). And it is about 365.25 days.

The length of a month is based on how long it takes the moon to orbit the earth (ditto for the earth/moon position historically). It is 27.32 days.

So a month does not divide exactly into a year; there are ~13.37 months in a year. It takes 52 years between the two matching up.

Historical inaccuracies, and the fact that some events were calculated on a yearly (Solar) calendar, while others were calculated on a monthly (Lunar) calendar, resulted in a designated "12 months per year". Since this doesn't match up exactly with the actual period of the moon, some months had extra days added (in fact, February is the most accurate, as its 28 days most closely matches the actual period of the moon).

There were other events in the evolution of the modern calendar, like the addition of August and July (named for Julius Caesar and Augustus Caesar) to an originally 10 month-long year, but the 12 months-per-year is the basic reason.