Numbers, please! Shortest month of the year – How February came to have 28 days

Our calendar is a multicultural mishmash: we have the Babylonians to thank for the day with 24 hours of 60 minutes each, divided into 60 seconds. The annual calendar with its 12 months and additional days in leap years is a gift from the Catholic Church, while the astronomical Julian calendar is a product of the scientific revolution.

We have a Roman king to thank for the strange position of February as a month with 28 days, which is given an extra day in leap years with a few exceptions.

Around the year 700 BC, Numa Pompilius reigned as the mythical king of Rome. He was the successor to the warlike Romulus, who had united various tribes into Romans, and took care of legislation and religion in peaceful times. Among other things, he revolutionized the Roman calendar with its 10 months and a year that began on 1 March, the day of Mars, the god of war. Numa Pompilius set the year at 355 days and added January and February to the number of months, as well as the leap month Mercedonius. The months, which previously had 30 days, were reduced to 29 days and February to 28 days.

Zahlen, bitte!

In this section, we present amazing, impressive, informative and funny figures ("Zahlen") from the fields of IT, science, art, business, politics and, of course, mathematics every Tuesday. The wordplay "Zahlen, bitte!" for a section about numbers is based on the ambiguity of the German word "Zahlen." On one hand, "Zahlen" can be understood as a noun in the sense of digits and numerical values, which fits the theme of the section. On the other hand, the phrase "Zahlen, bitte!" is reminiscent of a waiter's request in a restaurant or bar when they are asked to bring the bill. Through this association, the section acquires a playful and slightly humorous undertone that catches the readers' attention and makes them curious about the presented numbers and facts.

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After every four years, the leap month Mercedonius with 22 days was added. The poet Virgil described the background to this extensive calendar reform with “Numero deus impari gaudet”, “God rejoices in odd numbers”. Martius, Maius, Quinctilis (later Julius) and October were 31 days long, the other months 29 days, with the exception of Februarius, the month commemorating the dead (Parentalia) with 28 days.

Number types as gender types

In the developing Roman world, odd numbers were masculine and even numbers feminine. Historians such as Theodor Mommsen suspect an influence here from the Phytagoreans, who saw odd numbers as limited, masculine and happy, while they classified even numbers as unlimited, feminine and unhappy. This classification runs through Roman history, from the three-child law for Roman women and the five children for unfree women, from the Nones (7th or 5th day of a long or short month) to the Ides (15th or 13th day of a long and short month).

All Roman festivals were celebrated on odd-numbered days, which meant that celebrations lasting several days were interrupted on even-numbered days. When the Romans came to Egypt, they were disturbed by the regular calendar with 12 months of 30 days each, to which 5 extra days (or 6 in leap years) were added.

Numa Pompilius' classification lasted a remarkably long time. The first change came in 153 BC, when the beginning of the year was moved from March 1 to January 1. In addition to the counting of the days of the month, there was the system of market days (Nundinae), on which the rural population was allowed into the city and the city dwellers replenished their supplies. In 45 BC, the first fundamental calendar reform was finally introduced under Julius Caesar with a year of 365.25 days, the abolition of the leap month and the insertion of a leap day every four years after February 28, according to the formula 365+(1/4)-(1/000).

Gregorian calendar still valid today

However, the Julian calendar was still not exact enough and resulted in a calendar delay compared to the solar year. With the introduction of the Gregorian calendar on October 15, 1582, and the correction of 10 days that had to be skipped, we have already arrived at the calendar used today. Pope Gregory and his mathematical advisors defined the year using the formula 365+(1/4)-(1/100)+(1/400)=365.2425 days. The adjustment was made according to a new leap rule, because of which so-called secular years (they close the century and are therefore divisible by 100 without remainder), whose number divided by 400 does not result in a whole number, are not considered leap years.

Notwithstanding all the increasing precision, the February of Numsa Pompilius with its 28 or 29 days survived the passage of time until 1606, when the French calendar researcher Joseph Justus Scaliger proposed astronomical universal time and propagated it as the Julian calendar after his father Julius César Scaliger. It began on January 1, 4712 (corresponding to January 1, 4713, BC because there can be no year zero) and is still a useful method today, as each day and time has its own ordinal number into which it can be converted.

Apart from this calendar order, we still live with the Gregorian calendar and February with 28 days. With the founding of the League of Nations in 1920 and later the UN, there was an attempt to replace it with a world calendar. Like many other sophisticated calendar systems, it failed.

(dahe)