What is "red-letter days"

Usage examples of "red-letter days".

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.