Why does February, usually, have 28 days?

Why does February have 28 days?  The Origins in Ancient Rome and the Rivalry between Cesare and Augustus.

From ancient Rome to the October revolution, calendars have historically been revised, and February is the month that has undergone the most changes

"Thirty days hath September, April, June, and November. All the rest have thirty- one, Except for February…”  Thanks to the nursery rhyme we learned as children, it is easy to remember that the month of February is shorter than the others, and the only one to have 28 days (which become 29 only in leap years).  But what is the reason for this "anomaly"?  The subdivision of the months of the year dates back to ancient Rome, and the length of February may depend on a dispute between Julius Caesar and Octavian Augustus.

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The origin of calendars in Ancient Rome

An early version of dividing up the year is attributed to Romulus, the legendary founder of Rome.  There were only ten months (counted from March to December) and 304 days.  In fact, the winter period was not regulated because it was considered unimportant both from war and agriculture standpoints.  The first four months of the year were dedicated to the deities Mars, Aphrodite, Maia and Juno, while for the others, the order they followed in the calendar was validated later. 

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In 713 BC, Numa Pompilius, one of the seven kings of Rome, added the months of Ianuarius (January) and Februarius (February) to the end of the calendar.  It was therefore decided to dedicate the last month of the year to purification, in Latin februare, and to the Etruscan god Februus.  However, this division was imprecise.  In order to keep the calendar aligned with the seasons, an integrated month, the mercedonium, was added every two years.

The calendar of Julius Caesar

In 46 BC, Julius Caesar, as pope maximus, issued a new solar calendar developed by the Egyptian astronomer Sosigene of Alexandria.  It began to count the years from 1 January (instead of 1 March) and introduced the leap year in place of the interchangeable month.  One theory argues that, in the Julian calendar, February usually counted 29 days (30 in leap years). 

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When the Senate decided to dedicate the eighth month of the year to Octavian Augustus, someone pointed out that that month had only 30 days, while July, dedicated to Julius Caesar, had 31.  In order not to wrong the new emperor, it was decided to remove a day in February and add it to August. Other historians argue, instead, that from the beginning, February had 28 days and for this reason it was considered unlucky (according to the Romans, even numbers were a bad omen).

The Gregorian calendar (with some exceptions)

Julius Caesar's calendar was maintained until 1582, when Pope Gregory XIII introduced what was called the Gregorian Calendar in his honor.  A few changes were made, but February remained the only 28- day month.  This calendar is still used in most of the world today, although there have been a few exceptions throughout history.

In Sweden, for example, the calendar of 1712 marked the date 30 February  (in 1699, King Charles XII, had decided to conform to the Gregorian calendar, but had failed to implement the project).  

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In Russia however, with the October Revolution of 1917, it was decided to change the course of the months.  However, an error was committed and so, in the years 1930 and 1931, the month of February lasted 30 days.