Circa 50 BC, the Roman Astrologers realized, that their calculations, about the "length" of the earth-year, were amiss by about half a day... so "half a day" had to be accommodated "somewhere" in the year's length to make it a "proper" year.
The Roman Calculations giving the year' length at around 365.50 days ... were fine tuned over the years, and perfected, to a quarter of a day difference, over the originally held 365 flat ... days long year.
So came the 366 days long year ... where after every three 365 days long years came the Corrective 366 day long Leap Year.
Perhaps, in early Julian Calendars, the leap year came every second year.
The Interesting thing is ... That this leap and regular year is still under fine tuning ... This places the Zodiac in shambles and renders all Astrological Calculations and Claims WRONG.
Julius Caesar is credited with having introduced this innovation ... circa 46 BC.