I always wondered

Why do some months have 30 days and others 31?

October 4, 2026 · 2 min read

If someone had designed a calendar from scratch they would have made the months equal. Nobody did. Ours grew by repair, and the lengths are what survived.

The first Roman calendar

The earliest Roman calendar was only ten months long and began in March, with six months of 30 days and four of 31. Its leftovers are still in our month names: September means the seventh month, October the eighth, November the ninth and December the tenth, because they were counted from March.

Two months are added

Around the seventh century B.C., the story goes, King Numa Pompilius added January and February to fill the gap in winter. Britannica notes that the Romans considered odd numbers lucky, which is one reason the months were given odd lengths. February, at the end of the year and thought unlucky, was the exception.

Caesar tidies it up

That calendar kept drifting away from the seasons, and officials were known to add days to suit politics. In 45 B.C. Julius Caesar replaced it with a 365-day year with a leap year every four years, starting the year on January 1. To reach 365 days several months were lengthened, and most ended up alternating between 31 and 30 days, with February left over.

The August story

You may have heard that the emperor Augustus took a day from February so that August, named after him, would be as long as July, named after Julius Caesar. It is a popular story, but Britannica says it probably has no basis in fact. Historians also disagree about the exact month lengths in the early Julian years, so treat it as a story and not a record.

How the numbers work out

Twelve months of 30 days make 360 days, five short of a year. Seven months have the extra day, which makes 367, and February takes two away, giving 365. Seven months of 31, four of 30 and one of 28 is 217 plus 120 plus 28, which is 365.

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