Red Letter Day Meaning

(idiomatic) A particularly significant day or a day to celebrate (personal or sectarian).

Example:   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."