Phrase Demarking Special Date on Calendar, __ Day

This page is the CodyCross answer for the clue below from All Things Water, Group 1296, Puzzle 2 (Castle Moat).

Question is: Phrase Demarking Special Date on Calendar, __ Day

and the answer is: RED-LETTER

Word Meaning:

RED-LETTER — Particularly significant or positive.

Definitions from Wiktionary.