The first calendar to be widespread in Europe, thanks to the Roman Empire, was the Julian one. The calendar is named after Julius Caesar, who in 46 BC held the office of Pontifex Maximus. Among his duties there was precisely the definition of the annual calendar.
Caesar decided to reform the Roman calendar, choosing a solar-type, because the old Roman calendar had numerous defects due to its variability. It was corrected with the support of the Greek astronomer Sosigenes of Alexandria. The early calendar, called after Romulus, was made up of 304 days, organized into ten months, 6 of 30 days and 4 of 31. Until Caesar the Republican calendar had precisely the following structure: March, May, July and October lasted thirty-one days; February lasted twenty-eight days; the other months were of twenty-nine days.
The Month stated with the Calends, then the Ides in correspondence with full Moon were on the thirteenth day of the months, except for the long months (March, May, July, and October), when they were on fifth teen.
The Nones similarly were on the fifth day (the first quarter of the Moon), except for March, May, July, and October, when the Nones were on the seventh day.
The Roman calendar was counting the days backwards like a countdown. The days after Calends were enumerated as days before the following Nones; days after the Nones were numbered as days before the next Ides, and so on the days after Ides as the days before Calends. As example if the Ides of January were on 13th January, the day called "a.d.V. Id.Jan." was the fifth day a.d. (ante diem -"the day before") before Id = Ides, so the 9th January counting the day itself. Numa Pompilius, second king of Rome, added the months of January and February, which brought the year to 355 days and the month duration was of 28 up to 31 days.
Compared to the solar year of 365 days, these ten days were compensated with a thirteenth month (lasting alternative 22 and 23 days) every two years. This many approximations in the centuries caused a considerable lag. To put an end to this uncertainty, Caesar decreed that the year 46 BC should have lasted 445 days, called “annus ultimus confusionis”, the last year of confusion, to align with the sun. From then onwards, the year would last 365 days and every four years there was a leap year. This because the solar year duration of 365.23 days adds one day every four years.
The year of 366 days became the leap year with and additional day after February 22. The 12 months had a duration of 31 or 30 days except February. At Ceasar time the monthly structure was still based on Ides and Nones, kept at the same days, even if the days preceeding Kalends were more and only later the seven week was introduced (see next paragraph). Even the backward counting was kept.
This is the current calendar except few changes. The Julian calendar defines that the leap years are those multiples of four. The Julian year therefore lasts 365 days and 6 hours on average (three years of 365 days and one of 366), but the average solar year (see link) is shorter of 11 minutes and 14 seconds. Consequently, the Julian calendar accumulates about one day of delay in 128 years. From the Roman Empire until modern times this calendar was used before being replaced by the Gregorian calendar. Today, the Julian calendar is used mainly by Orthodox Christian churches, such as Russian, Ukrainian, Serbian, Macedonian, Georgian, Ethiopian and Church of Jerusalem. The difference lies in the different liturgical dates, such as Christmas, falling on 7th January in Julian calendar.
Image: Caius Julius Caesar - Remastered from Alfred von Domaszewski Geschichte der Romischen Kaiser Verlag von Quelle & Meyer in Leipzig 1914
Source: Wikimedia Commons