I find the idea of unlucky "egyptian days" mentioned again and again in relation to the julian calendar, but without much detail, beyond the fact that they're Bad, as if whatever they are is common knowledge and not worth explaining.
I'll be hauling my lazy behind over to the library to look their entry up in the annoyingly undigitized third volume of "Kulturhistorisk Leksikon for Nordisk Middelalder..." but thought maybe someone in here could help fill this embarrassing void for me. I originally ran into them while reading about Scandinavian runic clog calendars, but they seem to be an integral part of the julian system?
Back from the library! Turns out these are supposedly the dates on which God smote Egypt. Which should have been obvious, perhaps. They first turn up in the same 4th century source that's the earliest source of the birth of Christ. But while the reasoning behind calculating the nativity is known, I can't find anything on the reasoning behind the placement of the "Egyptian Days"...