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
[ 'red ] (adjective.) before 12th century. Middle English, from Old English rEad; akin to Old High German rOt red, Latin ruber & rufus, Greek erythros.