From N.S. Gill,
Your Guide to Ancient / Classical History.
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Definition: The Julian Calendar was a major change in the ancient calendar that was designed to realign the months of the calendar with the observable seasons. According to this Julian Calendar there were 12 months, 365 days, and leap years every four years. Julius Caesar introduced the reforms in 46 B.C. and the new Julian Calendar began the next year, in 45 B.C. Previous Roman calendars were 355 days long, with intercalated days between February and the beginning of the year, March. Priests had been determining when and how many days to add to the intercalary period.

According to the Julian Calendar, the new year started January 1.

For more details, see Who is the Julian of the Julian Calendar and what is the Julian Calendar?

Examples: The year before the Julian Calendar took affect was 445 days long in order to make subsequent years line up with the seasons.