One of the mysteries of the English language finally explained.
1A character or word written or printed in red, typically in order to draw attention to particularly important or significant information. Chiefly in plural.
2A small moth, Agonopterix ocellana (family Oecophoridae), of Eurasia and North Africa, having a white spot ringed with red in the centre of the pale forewing. Also "red-letter flat-body". rare.
Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator. From red + letter.
red letter/ˌrɛd ˈlɛtə/
In this article we explore how to impress employers with a spot-on CV.