red-letter day 
noun
A day that is pleasantly noteworthy or memorable: many a trout angler has had a red-letter day on Loch Awe...
- Because feast days in such almanacs and calendars were frequently written or printed in red, a red-letter day came to be a term for one that was special.
- Still, pub owners say, it was a red-letter day for business and they are planning a second block party for the first weekend of November.
- Today, as I said, is a blue-ribbon day, a red-letter day, or whatever one wants to call it.
Origin
Early 18th century: from the practice of highlighting a festival in red on a calendar.