ˌred-ˈletter day noun [countable]
informal a day that you will always remember because something special happened that made you very happy

[TahlilGaran] Dictionary of Contemporary English ▲

red-letter day
a day that is very important or very special.
The day our daughter was born was a real red-letter day for us.

[TahlilGaran] English Idioms Dictionary ▲

red-letter day
a day that is memorable because of some important event
It was a red-letter day when my sister received her graduation diploma.

Saturday was a red-letter day when we finally won the school championship.

[TahlilGaran] English Idioms Dictionary ▲

red letter day
n. phr. A holiday; memorable day (usually printed in red on calendars).
The Fourth of July is a red-letter day.
It was a red-letter day for Felix, when he won the lottery.

[TahlilGaran] English Idioms Dictionary ▲