The Collaborative International Dictionary
Red-letter \Red"-let`ter\ (-l?t`t?r), a. Of or pertaining to a red letter; marked by red letters.
Red-letter day, a day that is fortunate or auspicious; -- so called in allusion to the custom of marking holy days, or saints' days, in the old calendars with red letters.
Wiktionary
a. Particularly significant or positive.
Usage examples of "red-letter".
Henry Rogers in the village--in La Citadelle, that is--was a red-letter day.
The evening was, I sensed: for one thing, Calyxa announced then, at first augustly, that next day, the ninth since the sun's entry into Leo, was the twenty-fifth anniversary of her birth and the twentieth of another red-letter day on the calendar of her life, which she'd tell me about tomorrow.
He spun around only to watch helplessly as the heavy, muscular nurse ran out of his room and down the hall to a door beneath a red-lettered Exit sign.
Today was a red-letter day for him: two helpings for dinner, two helpings for supper.
The arrival of a life probe must be a red-letter day on any species’.
If I had the luck, certain mornings, to give up my seat in the bus or subway to someone who obviously deserved it, to pick up some object an old lady had dropped and return it to her with a smile I knew well, or merely to forfeit my taxi to someone in a greater hurry than I, it was a red-letter day.
The first time he did two complete lengths of the pool submerged was a red-letter day for Harry.
Sam was trying to remember what she'd expected to feel on this red-letter day.
For those who believe in omens this must surely be a red-letter day!
He would willingly have dispensed with the introductions which were forced upon him, but while Mrs Ditchling was cast into housewifely distraction by his visit, because she was afraid he would find the place a bit untidy -- which was her way of describing a scene of such chaos as might be expected to exist in a very small cottage inhabited by seven persons, most of whom were of tender years -- it was obviously considered by the rest of the family to constitute a red-letter day in their lives, Alfie, a young gentleman in velveteen knickers and Fair Isle jersey, going so far as to dash out into the garden at the back of the cottage yelling to his brother Claud to come quick, or else he wouldn't see the detective.
So wind and frost and snow, and perhaps a good stiff storm of hail, were Toby Veck's red-letter days.
These were red-letter days for them both--the sole bright spots in their lonely lives peopled by vain regrets.
Wednesdays and Saturdays came to be, indeed, red-letter days to her.