Each month in the modern Gregorian calendar consists of at least 28 days. That number would be a nicely rounded 30 were it not for February. While every month besides the second in the calendar contains at least 30 days, February falls short with 28 (and 29 on a leap year). So why is the most widely used calendar in the world so inconsistent in the lengths of its months? And why is February stuck with the fewest number of days? Blame it on Roman superstition.

The Gregorian calendar’s oldest ancestor, the first Roman calendar, had a glaring difference in structure from its later variants: it consisted of 10 months rather than 12. In order to fully sync the calendar with the lunar year, the Roman king Numa Pompilius added January and February to the original 10 months. When he reorganized the calendar’s dates to fit the new format, Numa tried to avoid having months that consisted of an even number of days, as Roman superstition held that even numbers were unlucky. But in order to reach the 355 days of the lunar year (354.367 to be exact, but he rounded up to keep it odd), 1 month out of the 12 needed to contain an even number of days. This is because of simple mathematical fact—the sum of any even amount of odd numbers will always equal an even number. So Numa chose February, a month that would be host to Roman rituals honoring the dead, as the unlucky month to consist of 28 days.

Despite changes in the calendar as it was altered after Numa’s additions—alterations that include the shortening of February at certain intervals, the addition of a leap month, and eventually the modern leap day—February’s 28-day length has stuck.