The main reason why the months have different lengths is the lunar cycle, which consists of 29.53 days. There are 365.25 days in a year, and this number can’t be divided by 29.53 evenly.

In the most ancient ancestor of the Gregorian calendar, which is the Roman calendar mentioned above, there were 10 months, not 12 months. 6 months with 30 days and 4 months with 31 days. The Roman king, Numa Pompilius (753–673 BC), added 2 more months to the original months: January and February.

At that time, there was a superstition in Rome that even numbers could bring bad luck, so Numa wanted to avoid even numbers appearing on the calendar. Numa decided that all 30-day months should become 29-day months. It helped to avoid bad luck, but not entirely.

There is a rule in math that the sum of an even number (12 months) of odd numbers will be even no matter what. Numa didn’t want the whole year to consist of an even number of days, so he decided to “sacrifice” February and make it have 28 days. February was decided to become a month when the rituals of veneration for the dead would be carried out since the month was obviously unlucky. As a result, Numa’s calendar consisted of 355 days.

As you know, the calendar was changed after that, but February retained the number of days assigned to it by Numa.

Julius Caesar was the one who changed the months to the way we know them now. Since 46 BC, the months began to have 31 and 30 days alternatively, except for February.

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