Augustine, The City of God 15.12
“Six multiplied by six, which makes the square of six, adds up to thirty-six days. When this is multiplied by ten, this makes three hundred and sixty days, which gives us our twelve lunar months. The solar year requires five more days and one quarter day to be completed, and this is why an additional say they call the bissextus is added every fourth year. So, additional days were added by ancient authorities to make the days align with the years. The Romans called these days intercalary.”
Sexiens autem seni, qui numerus quadratum senarium facit, triginta sex dies sunt, qui multiplicati deciens ad trecentos sexaginta perveniunt, id est duodecim menses lunares. Propter quinque dies enim reliquos quibus solaris annus impletur et diei quadrantem, propter quem quater ductum eo anno quo bissextum vocant unus dies adicitur, addebantur a veteribus postea dies ut occurreret numerus annorum, quos dies Romani intercalares vocabant.
Bissextus [or bissextile, bisextilis] was the name given in the Julian calendar because the day was added on the 24th of February in leap years as ante diem sextum Kalendas Martias or (a.d. vi Kal. Mart.), the twice-sixth day before the Kalends of March. Romans added time at the end of February because of a ritual accord following the winter solstice. Prior to the Julian calendar, when the year was set at 355 days, there could be an entire leap month added. According to our records, the calculations were so off in 45 BCE that Julius Caesar imposed 67 additional intercalary days.