The internal structure of the calendar

The ancient Roman calendar gave special names to two days in each month: the Kalendae (in English, “Calends,”)which was the first day of the month; and the Idus (in English the “Ides,”) which was the fifteenth day of March, May, July, and October, but the thirteenth day of every other month.  Other days were specified by counting the days until the next Calends or Ides.  So, the last day of April was pridie Kalendas Maias, the first day before the Calends of May.  There was some special significance to what came to be called the Nonae (in English the “Nones,”) that is to say, the ninth day before the Ides.  So, in March, May, July, and October, the Nones would fall on the seventh day of the month, and in other months they would fall on the fifth day.  So today, being the fifth, is the Nonae Decembris.   As far as the formal language of law and religion were concerned, this arrangement around the Calends and the Ides constituted the whole internal structure of the month.  The Romans did experiment with various forms of the week, most notably an eight-day week that determined when markets would be held.  Undoubtedly these sequences of days would also have influenced the Romans’ perceptions of time, even if they were not regularly integrated into the official calendar.

I bring this up because of an xkcd strip that appeared a week ago today.  Cartoonist Randall Munroe used Google’s Ngram search to tabulate the number of occurrences of each date by its name (ordinal number + month name) in English-language books since 2000.  In months other than September, the 11th is mentioned substantially less often than any other date.  It's been that way since long before 9/11 and I have no idea why.

His results suggest that our months do have some kind of internal structure that is not illustrated on our usual calendars.  Those simply display numbers in a grid of weeks.  Yet Mr Munroe’s findings suggest that there is more to it than that.  As the mouseover text points out, in eleven of twelve months the eleventh is mentioned much less often than any other date.  The exception is of course September, where references to the events of 11 September 2001 propel that date to the very top of the list of frequently named dates.  Yet this pattern was already well-established before 2001, and there is no obvious explanation for it.

Some variations in frequency are relatively easy to explain.  The first of the month is usually a day when many bills and reports are due, and so the first is among the most named dates of each month.  Holidays are also prominent; notice, though, that the eleventh of November, Veterans’ Day in the USA and Remembrance Day in the countries of the Commonwealth, is no bigger on Mr Munroe’s chart than the little elevenths of the other months.  The 15th of April is quite prominent; that has traditionally been the day when income taxes were due in the USA.  But, in addition to the mystery of the obscured elevenths, we also notice that the fourth and nineteenth are bigger than average in most months.  Why would that be?  Perhaps it doesn’t mean anything, but perhaps there is some explanation that would become obvious if we were in the habit of thinking of calendars, not as the grids of weeks that are usually tacked on walls in the West, but as structures built around major days, structures like those the ancient Romans used.  Too bad we can’t raise some ancient Romans from the dead and put them in charge of investigating the question, their perspective might result in a most fruitful study.  I suppose the best substitute would be classical scholars who have spent time studying the ancient Roman calendar.

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