Only slightly less well known is that the Roman Calendar was wonkily asymmetrical: front loaded with Kalends, Nones and Ides with a long void in the second half of each month until Kalends comes round again. It won't surprise anyone that the Roman month was originally based on the phases of the moon [moonth, if you will]. But, by the time of Caesar, the Romans were going some way to reconciling the incommensurate cycles of the Earth round its axis [days]; the Moon round the Earth [months]; and the Earth round the Sun [years]. And the Year triumphed over the month: it was more important to keep the seasons in sync - for farming and business - than to cede counting off the days to the phases of the moon. Islam still prefers to have Ramadan cycle through the solar year because 12 months of 29 or 30 days comes to 354 or 355 days not 365 or 366.
The Julian Calendar, implemented in 45 BCE, went for alternating 30 and 31 day months with a short February to make the sum up to 365 days. Every fourth year, February was 29 days long because the year is more-or-less 365¼ days long. It replaced a ragged-arsed 13 month year of alternating 29 and 31 day months totalling 355 days; with an intercalary month between Februarius and Martius every 2-3 years if it was religiously and politically expedient to reset the civil calendar with actual ecological reality. Julius Caesar, as Pontifex Maximus, recognised that having the Pontifex Maximus make the call on how many months there would be in a given year was wide open gate for corruption and finagling and so regularized the year to what we have today. I've been over how the Julian adjustment accumulated errors over the centuries and needed a rejig under the remit of Pope Gregory. Martius was the 1st month: which explains the names September, October, November, December not matching their position in the modern calendar. Also Blob Equinoxes: not always on the 21st.
Anyway, back to Ides! The Kalends (originally matching the New Moon) was the first day of each month. The last day of the month was Pridie Kalendas. Nones (originally marking the moon's first quarter) was on the 5th, except for Mar, May, Jul, Oct when it was two days later in the 7th. Likewise Ides normally fell on the 13th except for Mar, May, Jul, Oct when it was on the 15th. So Jules was done to death, by Et tu Brute and others, on the 15th March 44 BCE, he was 55.
The Romans were mad for counting backwards - see IV and IX and pridie kalendas above. There was a Pridie Nonas and a Pridie Idus as well. The other days were also looking forward quantitatively to the next Big Day:
- 1 Kalendis (Kal.)
- 2 ante diem quartum Nonas (a.d. IV Non.)
- 3 ante diem tertium Nonas (a.d. III Non.)
- 4 Pridie Nonas (Prid.Non.)
- 5 Nonas (Non.)
- 6 ante diem octavum Idus (a.d. VIII Eid.)