Why are calendar months different lengths?

The old Roman calendar is said to have begun the year in March. That is why September, the ninth month, comes from the Latin word for seven.

Later, January and February were added, making twelve months. The Romans considered even numbers unlucky, so they gave most months 29 or 31 days, but to make the total work, one month had to have an even number. That job fell to February, the month of purification rites, and it got 28 days.

That calendar had only 355 days, so it kept drifting away from the seasons. In 46 BC, Caesar stretched the year to 365 days by adding one or two days to the months that had 29. January, August and December became 31 days long; April, June, September and November became 30. Only February was left alone at 28.

The leap day added every four years also went into February, because the Romans had traditionally used February whenever they needed to slip in extra days.

A popular story says August has 31 days because Emperor Augustus did not want the month named after him to be shorter than July, so he took a day from February. Many historians, however, regard this as an explanation invented by the 13th-century scholar Sacrobosco.

August already had 31 days under Caesar, before it was renamed. Records say Augustus chose that month not for its length but because it was when he won his greatest victories.

If you forget which months have 31 days, make a fist and tap the knuckles and the dips between them, starting with January. Months that land on a knuckle have 31 days, months in a dip have 30, and February is the exception. When July lands on the last knuckle, August goes back to the first one, so July and August both have 31 days.

The same lengths matter for monthly passes and subscriptions: whether a plan lasts 30 days or one calendar month can change the day it ends.