As far back as the historical record can be traced, we know that certain days have been thought to be unlucky. In medieval times they were often listed in calendars as the dies Aegyptiaci, the Egyptian days, since they were supposed to have been identified by Egyptian astrologers, considered to be authorities on such matters. Some said they were days on which calamitous events had occurred in ancient Egypt, such as the plagues described in the Bible.
Medieval calendars precisely identified the days that were to be considered inauspicious, on which no project or enterprise should be begun: 1 and 25 January; 4 and 26 February; 1 and 28 March; 10 and 20 April; 3 and 25 May; 10 and 16 June; 13 and 22 July; 1 and 30 August; 3 and 21 September; 3 and 22 October; 5 and 28 November; and 7 and 22 December. It was considered especially important that doctors should not let blood on these days.
Another Latin term for them was dies mali, the unlucky, evil, or unpropitious days. By the beginning of the fifteenth century, the Latin phrase had been Anglicised into dismal, at first in direct reference to the evil days, but later to any event that brought misfortune and disaster. By the seventeenth century the word had weakened to our modern sense of something that merely causes gloom or depression. When in 1849 Carlyle described economics as “the dismal science”, he meant only that it was cheerless.