red-letter

The compound 'red-letter' originates from the practice, dating to medieval church calendars, of marking saints' days and important festivals in red ink, while ordinary days were printed in black. The term 'letter' in this context refers to the initial letter or the entire name of the feast day as it appeared on the calendar. Over centuries, the convention spread to secular almanacs and diaries, solidifying the association of the color red with special distinction. Thus, the literal meaning of 'marked with red letters' evolved into the modern figurative sense of any day or event that is memorably significant or auspicious.

Structure: red (color) + letter (character) - red: From Old English rēad, of Germanic origin, related to Dutch rood and German rot. Functions as an adjective modifying 'letter'. - letter: From Old English letter, via Latin littera ('letter of the alphabet', 'document'). Functions as a noun, here referring to a printed or written character.