Julius Caesar's astronomers explained the need for 12 months in a year and the addition of a leap year to synchronize with the seasons. Also original fifth and sixth months were renamed July and August in honour of Julius Caesar and his successor Augustus. Their days have also been increased each to 31 days.
Similarly, you may ask, when did calendar change to 12 months?
In 45 B.C., Julius Caesar ordered a calendar consisting of twelve months based on a solar year. This calendar employed a cycle of three years of 365 days, followed by a year of 366 days (leap year). When first implemented, the "Julian Calendar" also moved the beginning of the year from March 1 to January 1.
Also to know, was there a year 0?
Year zero. The year zero does not exist in the Anno Domini (AD) system commonly used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year 1 BC is followed by AD 1.
When did Year 1 start?
In some ways, yes. When Julius Caesar introduced his calendar in 45 B.C.E., he made 1 January the start of the year, and it was always the date on which the Solar Number and the Golden Number were incremented.
What did they call years before BC?
The references AD and BC are sometimes replaced by CE and BCE: Common Era and Before the Common Era. The Roman calendar was counted Ab urbe condita ("from the foundation of the city"), in 753 BC; and it continued in use until the Anno Domini calendar was introduced in AD 525.