All the calendars of ancient times, though some of them were quite successful, had a number of shortcomings. In practice, they created a lot of complications. They were not due to a lack of knowledge. Ancient astronomers knew, as we know, the true length of the tropical year, but various considerations - mostly religious ones - prevented the necessary reform from being undertaken. The calendar was something sacred. Breakdown of the year into months, decades, the weeks were also sanctified by tradition. Life, however, required order. Julius Caesar understood this (100—44 p.n.e.) deciding on a general change.
The calendar reform was commissioned by Caesar to a group of astronomers - headed by Sozygenes of Alexandria. The Egyptian calendar with the Sotis year was adopted as the basis of the new calendar, fixed on 365,25 day. The "rest" was actually Egyptian too: 12 months and leap year (introduced, as we know, by Ptolemy III Eurgetes). However, some features of the native calendar were also preserved.
The disproportions between the Roman calendar and the solar year were already considerable. The difference was 90 days. What was done with her? Just: it was added to the year before the reform. It was done as follows: until February, which was a leap month in the Roman calendar, added 23 days, others 67 while - until the last month. Year 46 p.n.e. so he counted 445 days.
The new calendar started functioning from 1 I 45 r. p.n.e. (and according to the then count of lats - v 708 r. since the founding of Rome). It covered a 4-year cycle: three years counted after 365 days, fourth - 366. The year was divided into 12 months, whose names remained unchanged. Only their length has changed: even numbers counted after 30, odd - after 31 days, except February, which he normally had 29, in a leap year while - 30 days, except that that extra day was added, like in the past, after 23 II. The extra day had no name, nor the numbering; it was generally called dies intercalaris, possibly - bissextus (because in the Roman calendar it was marked as the double VI March calendars).
After Caesar's death in 44 r. p.n.e. the name of the month of July (quinctilis) changed to Iulius, after Emperor Augustus Octavian took power, the name of August was Augustus. I… a certain order was disturbed in the calendar. August (formerly - Sextilis) was an even month and had 30 days. As the month of a "divine" ruler, however, it could not be worse than the month of Caesar. Therefore, its length was increased to 31 days, taking that day, of course, February (Februarius was unlucky with the Roman calendar, there were always tricks with him). Thus February counted normally now 28 days, and in leap years - 29. The increase in August also changed the numerical value of the following months. They were as follows now:
Ianuarius - January - 31 days
Februarius - February - 28 (29) days
Martius - March - 31 days
April Fools - April - 30 days
More - Maj - 31 days
June - czerwiec - 30 days
Iulius - July - 31 days
Augustus - August - 31 days
September - September - 30 days
October - October - 31 days
November - November - 30 days
December - December - 31 days
And this order of things has remained to this day - despite the later Gregorian reform, which, however, did not change the structure of the Julian calendar.