- Reckon from one of three fixed days in each month: Kalendae (Kalends), Nonae (Nones), Idus (Ides). The Kalends are always the first day of the month. In most months, the Nones are the fifth and the Ides are the 13th. In March, May, July, and October, the Nones are the seventh and the Ides are the 15th.
- Always reckon days leading up to the Kalends, Nones, or Ides.
- So, today (May 15, 2024) is the Ides of May, or in Latin: Idus Maiae [Idus is a fourth-declension plural feminine form.] Tomorrow, however, will be the 17th day before the June Kalends (counting, as the Romans did, inclusively).
- The year, in Christian reckoning, is anno Domini (ablative of annus = “in the year”, genitive of Dominus “of our Lord”), and then the ordinal number for the year, i.e., bis millesimo vicesimo quarto or MMXXIV.
- Full form of today (the May Ides): Id. Mai. A.D. MMXXIV (or, without abbreviations: Idus Maiae anno Domini MMXXIV, but use the ablative of Idus Maiae to indicate that something is happening, etc. “on the May Ides”, so Idibus Maiis.
- Full form of tomorrow (17th day before the June Kalends): a.d. XVII Kal. Iun. A.D. MMXXIV (unabbreviated: ante diem septimum decimum Kalendas Iunias anno Domini bis millesimo vicesimo quarto)
Etymologies 