Noun

1. A particularly significant day (personal or sectarian), usually very positive, sometimes very negative.

   Monday was a red letter day for her. She accomplished a lot and had fun doing it.

   We saw losses for the days in a row, but Black Tuesday was the worst red letter day of them all.

   • 1975, John Ankenbruck, Twentieth Century History of Fort Wayne - Page 286

     "In the meantime, the St. Valentine's Day Massacre at Chicago was a red-letter day in the gangster wars."