The shortest February.

 The smallest of the twelve months of the year is February. Other months are set to 30th or 31st, but February is only 28th and 29th even if the leap year returns. Why is February the shortest out of many months?

The Roman calendar initially had only 10 moon names from March (January) to December (October). The two months of November and December were ignored without a name, but the Romans were not very uncomfortable because it was during the agricultural cold season. Around the 8th century B.C., King Numa Pomphilius felt the need for a proper calendar and devised a new calendar.

Numa has set a year at 355 days. It was tailored to the movement of the moon. Numa added two months of January and Beverly to create a 12-month system in November and December when they were empty. The Romans believed that even numbers were unfortunate numbers, so Numa set 31 days for seven out of twelve months and 29 days for four months, respectively. As a result, a 28-day even month was inevitably needed to fill 355 days a year. Numa chose February, the last month of the year and one of the winter months. It was centuries later that January and Beverly were renamed January and February, the beginning of the year.

One theory says that February has become smaller as the Romans took one day out of February to increase August, which was originally scheduled for the 30th, to the 31st. This is because August, named after Emperor August, regarded it as profane for only 30 days, but it is not very reliable. There are several more theories about the origin and transformation of Pebruary, but one thing is clear that the Romans thought very undesirable of the short month.