This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by
2.1.1. What years are leap years? (Julian calendar)
The Julian calendar has 1 leap year every 4 years:
Every year divisible by 4 is a leap year.
However, the 4-year rule was not followed in the first years after the
introduction of the Julian calendar in 45 BC. Due to a counting error,
every 3rd year was a leap year in the first years of this calendar's
existence. The leap years were:
45 BC(?), 42 BC, 39 BC, 36 BC, 33 BC, 30 BC,
27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC,
AD 8, AD 12, and every 4th year from then on.
Authorities disagree about whether 45 BC was a leap year or not.
There were no leap years between 9 BC and AD 8 (or, according to some
authorities, between 12 BC and AD 4). This period without leap years
was decreed by emperor Augustus in order to make up for the surplus of
leap years introduced previously, and it earned him a place in the
calendar as the 8th month was named after him.
It is a curious fact that although the method of reckoning years after
the (official) birthyear of Christ was not introduced until the 6th
century, by some stroke of luck the Julian leap years coincide with
years of our Lord that are divisible by 4.