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the lands of the Roman Empire, its successor states, and their possessions, as well as other Christianized lands, until superseded by the Gregorian calendar, which was issued by Pope Gregory I in 1582.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.

Calendário juliano