To be more exact on what a red letter day is:

In missals and prayer books, the Church Kalendar is usually found in the front. Major feast days are shown in red, lesser feasts/observances in black type. Just as, in the text of a rite, such as the Mass, the words to be spoken are in black, the directions are in red, and are called 'rubrics', a word which has been taken up in secular usage. A rubric can mean an aside, or an instructional or clarifying note to a text.