Julian calendar

In 46 bce Julius Caesar, with the advice of the Alexandrian astronomer Sosigenes, created the Julian calendar by reforming the Roman calendar. The reforms included the following steps:

a one-time addition of 67 days between November and December to the year we call 46 bce. This unique 445-day year brought the seasons back to their traditional dates.
abandoning the old calendar's lunar aspects by establishing a fixed length for each month, in a regular pattern. With the exception of February, the months were alternately 31 and 30 days long.

Name of Month	Number of Days
Januarius	31
Februarius	29 or 30
Martius	31
Aprilis	30
Maius	31
Iunius	30
Quintilis	31
Sextilis	30
September	31
October	30
November	31
December	30

To make the length of the calendar year the same as that of the tropical year, every four years an extra day was to be added to Februarius between its 23 and 24 days, giving it 30 days. This made the average length of the year 365¼ days, which was pretty close to the actual length of the tropical year at that time.

The new calendar confirmed the use of the era of the founding of the city, ab urbe condita, introduced by Varro in the 1st century bce. This was usually taken to be 753 bce.

Changes in the Julian calendar after Julius Caesar's reforms

In 44 bce, at the instance of Mark Antony, the month Quintilis was renamed Julius in honor of Julius Caesar.

At some later point, probably by the consuls Asinius Gallus and Marcius Censorinus in 7 bce, the month Sextilis was renamed Augustus in honor of Augustus Caesar, Julius Caesar's successor. Since it wouldn't do to have Augustus's month shorter than Julius's, a day was taken from Februarius and added to Augustus. Unfortunately, that led to three 31-day months in a row (Julius, Augustus, September), so the pattern of alternating 31 and 30 was shoved forward by a month, restarting in Augustus. September and November were each shortened by a day, and October and December lengthened. To this day the months retain the resulting numbers of days, preserving the strange midsummer blip introduced in homage to the first two Caesars.

Name of Month	Number of Days
Januarius	31
Februarius	28 or 29
Martius	31
Aprilis	30
Maius	31
Iunius	30
Julius	31
Augustus	31
September	30
October	31
November	30
December	31

Later attempts by the emperors Caligula and Domitian to name months after themselves were annulled after their deaths. The emperor Tiberius, who opposed cults of personality, vetoed a proposal that September and October be renamed Tiberius and Livius after himself and his wife.¹

The use of the Christian Era with the Julian calendar did not begin until the 8th century, and then only by scholars; everyday use came centuries later.

The Julian calendar was gradually replaced for almost all purposes by the Gregorian calendar, beginning late in the 16th century.

1. Suetonius, Lives of the Caesars: Tiberius 26.