Definition of red letter day in English English dictionary
A particularly significant day (personal or sectarian), usually very positive, sometimes very negative
In the meantime, the St. Valentine's Day Massacre at Chicago was a red-letter day in the gangster wars..
very special day, special occasion, auspicious day, day that is worthy of notice; important day that will always be remembered; day in a diary of a woman which is marked with a secret mark to indicate the start of her menstrual period
A red-letter day is a day that you will always remember because something good happens to you then. a day that you will always remember because something special happened that made you very happy