1
general::
A special occasion, as in When Jack comes home from his tour of duty, that'll be a red-letter day. This term alludes to the practice of marking feast days and other holy days in red on church calendars, dating from the 1400s. [c. 1700]
American Heritage Idioms
2
general::
Phrase(s): a red-letter day
Fig. an important or significant day. (From the practice of printing holidays in red on the calendar.) • Today was a red-letter day in our history. • It was a red-letter day for our club.
McGrawhill's American Idioms And Phrasal Verbs