Translator´s Note that may help English- speaking readers to understand this text better: leap-year in Latin and modern Latin languages is called “bi sextum” year, like “twice sixth” year. This, as you will check later, is essential to be taken into account before starting to read this article in order to be able to understand it properly.
The knowledge of the Roman calendar is really difficult, what, at this point, readers have surely already realized as you read previous articles. Its important religious function, its primitive lunar nature, its first duration of ten months and later twelve, the influence of neighboring people such as the Etruscans, forced the Romans to make several reforms in their calendar which remained curiously archaic elements.
Particularly complex, as it has also been seen in another article, is the naming system they had for the days of the month. It should be remembered now that article that was already published and entitled Kalendae, Idus, Nonae: a very complex way to name the days of the month http://www.antiquitatem.com/en/ides-march-calendar-month-caesar-eve .
Let us briefly remember that the month is divided into three parts and they set the moments or time markers that make this division: the kalends that call on day 1 of each month, the nones that call on day 5 of the months of January, February, April, June, August, September, November and December and on day 7 of the months of March, May, July and October, and the ides that point on the 13th of the months whose nones fall on day 5 and that point on the 15th of those whose nones fall on day 7. Precisely Caius Iulius Caesar, Julius Caesar, was killed on the Ides of March, I mean, on the 15th of March, in 44 BC. The other days were called counting the remaining to the next moment or milestone.
We must also remember how in the calendar reformed by Julius Caesar, of solar character, the year has 12 months of different lengths, and a total of 365 days. In order to correct the gap or discrepancy with the tropical or solar year a day is added every four years.
Well, that day is added in February, behind the 24th, precisely because at that time the festivities called "Terminalia" finished, with which they celebrated the end of year (remember that in the beginning the year began in March) .
In the Roman naming system of the days of the month the 24th was "the sixth day before the Kalends of March”, in Latin, "ante diem VI (sextum) Kalendas Martias". The day added now every four years is called "ante diem bis VI sextum Kalendas Martias", which means the "twice sixth day before March 1st." From this name "bis sextum" comes the word "leap" year (in Latin languages “twice sixth” year). For example, in Spanish it is called “año” (year) “bisiesto” (two times sixth).
This is interesting but definitely complicated. As it usually happens with many words, we go on using them with a specific meaning, although they have lost the original reference that gave them meaning and sense, reducing them to a mere label word.