The phrase "red letter" typically refers to a special or important day or event, but what about its antonyms? Instead of a significant day, we could refer to an ordinary or mundane day. "Black letter" could suggest a negative or unfortunate event, while "gray letter" might mean a neutral or insignificant one. Other antonyms could include "unremarkable," "unnoteworthy," or "ordinary," all of which imply a lack of importance or significance. These words provide a contrast to the excitement and significance of a "red letter" day and can be used to describe less memorable events or days.