The phrase ‘Red-Letter Day’ can be used to signify a special or memorable day, a day of importance, such as a holiday, birthday, anniversary etc.
Example of use: Jacob — “I finished my last exam yesterday. From now on you can call me a college graduate.” Josh — “Well, yesterday was truly a red-letter day for you.”
The first recorded use of the phrase was in 'The book of Eneydos' printed in 1490: "We write in our calendars the high festies with red letters of color." However, the phrase goes back to the late 1300s when important and religious days were marked in red ink in calendars made in monasteries and convents. The red ink enabled them to notice the days, so that they could start preparing for them.