Here are shots of the pages from the 2005 Martyrologium Romanum concerning 29 February and Leap Year.
You can click these for large images.
You will have noticed the word The word bissextus. The mighty Lewis & Short says that bisextus, is "an intercalary day; so called, since the 24th of February = VI. Cal. Mart., was doubled".
In the Roman calendar the sixth day before the Kalends, or Calends, of March (24 February, counting back from the beginning of March) occurred twice in a leap year. So, bisextus (from bis "twice" and sextus "sixth") "the sixth day for the second time".
Just to expand this a little, again we look at the Martyrologium Romanum for 24 and 25 February.
On 24 February we have the sixth day before the Kalends of March.
On 25 February, we have this:
"The fifth day before the Kalends of March; or on leap year: the sixth day before the Kalends of March".
Looked at this way, the real "leap day" for the ancient Romans was at 24/25 February, not the 29th!
Remember, the Romans made their dates by counting forward to fixed points, the Kalends (1st day of a month), the Nones, (the 5th or 7th, depending on the month) and the Ides (the 13th or 15th depending on the month). There is a little verse to help us remember how this works:
In March, July, October, May
The Ides fall on the fifteenth day
The Nones the seventh; all besides
Have two days less for Nones and Ides.
Again, the Romans counted forward, rather than backward, like we do. We today think in terms of the first day of the month, and then the 2nd day from the 1st, and the 3rd from the first: our reference point is backward. The Romans counted forward to their days. and they also counted inclusively, counting the days themselves rather then the days remaining until the Kalends, Nones or Ides. Thus,
29 February = pridie Kalendas Martii – the day before the Kalends of March (which was the 1st day of the ancient Roman year for centuries!)
1 March = Kalendis martii – the Kalends of March
2 March = sexto Nonas martii – the sixth day before the Nones of March (which is 7 March)
3 March = quinto Nonas martii – the fifth day before the Nones of March
4 March = quarto Nonas martii – the fourth day before the Nones of March
5 March = tertio Nonas martii – the third day before the Nones of March
6 March = pridie Nonas martii – the day before the Nones of March
7 March = Nonis martii – the Nones of March
The Romans had abbreviations for dating. For example: a.d. for ante diem = "days before" and prid. for pridie, "the day before".
It is possible that the month February is named for a thong of goatskin called a februa from the verb februo ("to purify, expiate"). On 15 February the ancient Romans had a festival called the Lupercalia. Lupercalia is from the name of the Lupercal ("she wolf") cave on the Palatine Hill, sacred to the Lycean Pan. The Lupercal was recently rediscovered, btw., on October 2007, between the Temple of Apollo Palatinus and the Basilica of Santa Anastasia (where the Station of the 1st Mass of Christmas is). They are still searching for the entrance to the grotto, but they got photos of the inside with a remote device. Very cool stuff.
The Lupercalia were observed from 13-15 February. It was a festival for purification of the city from bad spirits and to promote fertility. At the beginning, two male goats and a dog were ritually slaughtered. Then two patrician men who belongeds to special priestly colleges, smeared with the blood wiped from the blade of the knife with wool soaked in milk. They then had to laugh (which would be my first reaction, I can tell you). After a feast, the two patricians, or luperci, cut thongs, februa, from the skins of the animals and put the bloody skins of the goats around their loins. They then had to run around the walls encircling the Palatine Hill, striking the people who lined their course with the thongs. This februatio was especially beneficial for girls and women, apparently, in promoting fertility.
It will be no surprise that these ancient rites were ended by Pope Gelasius (+496) of the Gelasian Sacramentary fame. Gelasius even had fight with a senator named Andromachus who wanted to keep it going, telling him that if we wanted the Lupercalia to continue, then Andromachus should be the one to run nude around the Palatine with only a bloody goatskin around his waist.
But I digress…