“a stone calendar that was displayed for public use. The parapegma was inscribed with a list of star phases and corresponding weather predictions… The stone parapegma had holes beside the inscription, in which a peg could be inserted next to the appropriate day. The peg was placed in the hole next to the day named in the calendar and holes were provided in order to count out each day, so that one would not lose the place in the month.” — Ancient weather calendars
It sounds like the original parapegma had multiple holes and pegs, and was used like a cribbage board to display the date. Later, people called a written version a parapegma as well. This was “precursor to the almanac.”
An example is the calendar/parapegma of Antiochus of Athens, from around 200 AD. Roger Pearse has put up a very accessible translation along with several related posts, available at the link just above.
It’s intriguing stuff. I’m surprised Neal Stephenson didn’t work a parapegma into Anathem somehow.