Only three days of each month in the Roman calendar had specific names. The first of the month was called the
Kalends. The Kalends were always on the first, but the other two days (
Nones &
Ides) depended on what month it was. For the months of
March,
May,
July, and
October, the Ides fell on the 15th and the Nones were on the 7th. For all other months, the Ides were the
13th and the Nones the 5th.
If it wasn’t one of these "special" days, a Roman would say it was n days before the next special day. The Nones always came 9 days before the Ides of a month, and 15 - 9 + 1 (the Romans counted inclusively) = 7. They always said that it was n days before a specific date, never after, so June 14 would be the eighteenth day before the Kalends of July (ante diem duodevicesimum Kalendas Quintiles). The Latin can be abbreviated a.d.XVIII Kal.Quin. (a.d. meaning in this context ante diem, not anno domini, of course!) The months are adjectives and describe the Kalends, Ides, or Nones when they are translated into Latin. They are Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis (later Iulius), Sextilis (later Augustus), September, October, November, December, and Mercedonius (also known as Intercalaris, a month inserted when the 355 day calendar was not in synchronization with the seasons - cool, huh? This is the klind of thing that the calendar reform people would not go for).
There are several days in a month that aren’t what you would expect. March 13 is described as 3 days before the Ides of March (as normal), but March 14 is the day before the Ides of March (pridie Idus Martias). The abbreviations for these are pr.Kal., pr.Non., and pr.Id.
Another strange name occurs only in a leap year when February 24 is counted twice. The first is a.d.VI Kal.Mart., but the repeated day is a.d.VI Kal.Mart.bis.