Julian calendar

The Julian calendar was issued by (and named for) Julius Caesar in 46 BC to standardize the Roman year, which had had a complicated and disruptive method of keeping it aligned to the seasons, resulting in severe misalignments, especially in times of war. Use of this calendar prevailed in the lands of the Roman Empire, its successor states, and their possessions, as well as other Christianized lands, until superseded by the Gregorian calendar, which was issued by Pope Gregory I in 1582. Adoption of the Gregorian calendar throughout Christendom was slowed by the Reformation and the Great Schism (Russia did not adopt it until 1917).

The system for accommodating the uneven number of days in a tropical year is very simple: every fourth year, a single day is added to February, the shortest month of the year.

Number of the month (modern)	Name in 46 BC.	Name after 44 BC. (named after Julius Caesar)	Name after 8 AD. (tribute to Emperor Augustus)	Number of days before 45 BC.	Number of days in 45 BC.
1	Januarius	Januarius	Januarius	29	31
2	Februarius	Februarius	Februarius	28	28
3	Martius	Martius	Martius	31	31
4	Abrilis	Abrilis	Abrilis	29	30
5	Maius	Maius	Maius	31	31
6	Junius	Junius	Junius	29	30
7	Quintilis	Iulius	Iulius	31	31
8	Sextilis	Sextilis	Augustus	29	31
9	September	September	September	29	30
10	October	October	October	31	31
11	November	November	November	29	30
12	December	December	December	31	31