Skip to main content

Why do the number of days in a month vary?


The first people to measure a year with any exactness were the ancient Egyptians. They started with a lunar (monthly) calendar based on the new moon's appearance every twenty-nine or thirty days. It was not very accurate.

The Romans also had a lunar calendar, and to make it agree with the solar year they added extra months whenever they needed to. Finally Julius Caesar adopted a new calendar based on a solar year of 365 days. Various changes had to be made in the months to make this calendar right. Here are the changes made by Caesar and others in the days of the month.

January was originally the eleventh month and had twenty-nine days. Caesar made it the first month and gave it thirty-one days. In Caesar's calendar February had twenty-nine days, with thirty in leap years. Emperor Augustus took one day from it and added it to August.

The number of days in March was always thirty-one. April, as a lunar month, had twenty-nine days. Caesar added a day to give it thirty. May always had thirty-one days and remained unchanged by Caesar. June had twenty-nine days and Caesar gave it thirty. July (named after Julius Caesar) was given thirty-one days by him.

When August was a lunar month, it had twenty-nine days. Caesar gave it thirty. Emperor Augustus, who named it after himself, added the day he took from February to make it equal to Caesar's month. September had twenty-nine days as a lunar month. Caesar gave it thirty-one, but Augustus reduced it to thirty.

October, in Caesar's calendar had thirty days, but Augustus gave it thirty-one. November also had thirty-one days under Caesar, and Augustus reduced it to thirty. December originally had twenty-nine days, Caesar gave it thirty, and then Augustus added another, giving it thirty-one.

The important thing for an accurate calendar was to have 365 days. The number of days in each month, as you can see, was decided first by Caesar and then by Augustus for whatever reasons they wished. We took our calendar from the Romans.

Related Articles