How do we know that Kalendae is the first day of a month?

Question

I have been told that Kalendae is the first day of a month. However, the Latin dates — which was discussed in this other question — alone do not make this obvious. Dates are expressed by comparison to three special days every month: Kalendae, Nonae, and Idus.

Based on everything I know about the Roman calendar, it would make sense that Idus was actually the last day of a month. Or perhaps days between Idus and the following Kalendae belong to no month at all. Or maybe a month consists of the three special days and other days have to be expressed in relation to these interesting days because other days belong to no month at all. It is not obvious that every day should be part of some month.

Is it clearly stated somewhere in ancient literature that Kalendae is the first day of a month? Is there some ancient source describing the structure of a month in this sense? Evidence from inscriptions is also welcome if it sheds some light on the matter. There seems to be a very strong modern consensus that Kalendae is the first day, but I realized that I know no reason for this.

Answer 1

In classical times the seven-day week was unknown; obviously, there could be no named days of the week to use as reference points. Months at least were of specified lengths, but the actual date was described by a clumsy method which depended on three datum points within the month itself. These points were the Kalends, Nones and Ides, which occurred in that order.

Dates were calculated by counting backwards from these datum points: that is to say, they were expressed as, for example, the third day before the Nones of January, the fourth day before the Ides of March, and the seventh day before the Kalends of November.

By long tradition, the Kalends were the first day of a month. There are many ways of demonstrating this, but one of the best authorities is the poet Ovid’s poetical treatise on the Roman calendar. It was written in strict chronological order, beginning in Book I with the first of January, which he calls the Kalends of January, named after Janus:

Ecce tibi faustum, Germanice, nuntiat annum / inque meo primus carmine Ianus adest.

Dates follow in the sequence identified first, by the Nones, secondly, by the Ides, and finally by counting back from the next Kalends — of February.

Book II begins:

Ianus habet finem. Cum carmine crescit et annus: / alter ut hic mensis, sic liber alter eat.

This leaves no room for doubt that the Kalends mark the first day of a month.

Answer 2

First point, the meaning of the Latin dates was never forgotten but traded continously (e.g. by the Roman Catholic church) into our times.

A second point is that we are able to verify some dates referring to astronomical events (e.g., solar eclipses) independently. Using such methods helped in falsifying the Phantom time hypothesis (aka Erfundenes Mittelalter.