red-letter

[ red-let-er ]

/ ˈrɛdˈlɛt ər /

marked by red letters, as festival days in the church calendar.

memorable; especially important or happy: a red-letter day in his life.

Origin of red-letter

late Middle English word dating back to 1400–50

Turns out, after all, to be a red-letter day or, that is, a brown-liquid evening.

The Ridiculousness of Father's Day |P. J. O’Rourke |June 15, 2014 |DAILY BEAST
When alluding, in after years, to this visit, Browning always spoke of it as one of the red-letter days of his life.

Life of Robert Browning |William Sharp
Love can always find some way to keep the red-letter days of its calendar.

The Little Colonel's Holidays |Annie Fellows Johnston
Sheep-shearing day continued the red-letter day on Nantucket well into the present century.

Nooks and Corners of the New England Coast |Samuel Adams Drake

January 24, 1848, was the red-letter day in California's history, and the news that transpired that day electrified the world.

Wealth of the World's Waste Places and Oceania |Jewett Castello Gilson
It was one of the red-letter days of my life, the day I owned a watch.

Life on the Stage |Clara Morris