We have a colleague who has red-sweater days, which led Moondog to wonder what the origin of the expression red-letter day was.
According to the Mavens:
The expression comes from the long-established practice of using red ink to indicate holy days in ecclesiastical calendars. A red-letter day–literally a day written in red letters–was a holy day, or church festival, and thus came to mean ‘any memorable or happy day’.
So a red-letter day is a good one. Unless you’re Hester Prynne, I guess.