A red-letter day is a day of special significance for some reason or another. Why do we call it a red-letter day?
This one is actually surprisingly straightforward. In Ancient Roman calendars, significant dates were marked in red, just as they often are today. In medieval manuscripts, initial capital letters and highlighted words (known as rubrics) were usually written in red ink.
There is a more specific type of day known as a scarlet day. This is a term used by the University of Cambridge to denote days on which Doctors are required to wear the full festal form of their academic dress, most of which feature scarlet robes.
And that’s pretty much it really. Usually there seems to be some kind of complicated history behind our simplest words. But in this case it’s very straightforward. For as long as we’ve been marking days, we’ve needed to highlight special days, and red tends to stand out, so we use that.
Sometimes it’s just that easy!