Egyptian days were one of the most enduring forms of calendrical prognostication in the ancient and medieval worlds. The Romans called these calendrical omens evil, dark, or ominous days (dies ægri, atri, mali, maledicti, ominosi, infortunati, and tenebrosi), and dies aegyptiaci at least by the fourth century C.E. There were twenty-four Egyptian days, two each month, recurring annually. In time, each day was paired with a particular hour viewed as dangerous, suspect, or inauspicious (hora suspecta, aegra, mala, timenda, or even unica). People who feared Egyptian days were bowing to the weight of tradition, much as someone today might have a superstitious fear of Friday the 13th. This article will focus on the role of cultural memory in providing a rationale for a belief in Egyptian days, and on mnemonic aids that were used to remember where they would fall during the calendar year. Special attention will be devoted to an obscure set of mnemonic verses with the incipit Armis gunfe, which the astronomer Johannes de Sacrobosco (Sacro Busto) (ca. 1195–ca. 1256?) disseminated in Paris around 1235. We will consider how these verses circulated in writing and were applied in practice.