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The Roman Days
 
During the Republic and the beginning of the Empire, the year was divided in eight-day weeks (marked A-H on the calendars) with a market day (nundina) every ninth day. In perpetual calendars, the letters A-H would start with A on January 1 and continue until December 31, which means that the market day (nundina) would fall on a different letter each year. We will set the letter A as the market day for 1997.
 
The first day of the month was called calends (Kalendae); the 7th (in March, May, July, October) or the 5th (the other months) day was called the nones (Nonae) and the 15th (in March, May, July, October) or the 13th (the other months) day was called the ides (Idus). The other days of the month were called by the number of days to the next the calends, nones, or ides (including the day of the calends, nones, ou ides itself): 3rd day before the nones, 7th days before the ides, 4th day before the calends of the next month. As the calends, nones, ou ides day itself was counted as "day 1", the day before last in April was called ante diem tertium kalendas Maias (3rd day before the calends of May). The last day of the month was called the eve of the calends: pridie Kalendas. Similarly, there was pridie Nonas and pridie Idus.
 
Examples ‹ January 2nd: ante diem quartum nonas Januarias; February 7: ante diem sextum idus februarias; April 15: a. d. septimum decimum kalendas Maias. The Latin ordinal adjective (in the accusative case) you need to form the name of all the days are the following:
19° : undevicesimum (XIX)
18° : duodevicesimum (XVIII)
17° : septimum decimum (XVII)
16° : sextum decimum (XVI)
15° : quintum decimum (XV)
14° : quartum decimum (XIV)
13° : tertium decimum (XIII)
12° : duodecimum (XII)
11° : undecimum (XI)
10° : decimum (X)
9° : nonum (IX)
8° : octavum (VIII)
7° : septimum (VII)
6° : sextum (VI)
5° : quintum (V)
4° : quartum (IV)
3° : tertium (III)
1° : pridie (PR)