A red letter day (sometimes hyphenated as red-letter day) is any day of special significance.
The term originates from Medieval church calendars. Illuminated manuscripts often marked initial capitals and highlighted words in red ink, known as rubrics. The First Council of Nicaea in 325 decreed the saint's days, feasts and other holy days, which came to be printed on church calendars in red. The term came into wider usage with the appearance in 1549 of the first Book of Common Prayer in which the calendar showed special holy days in red ink.
Many current calendars have special dates and holidays such as Sundays, Christmas Day and Midsummer Day rendered in red colour instead of black.
On red letter days, judges of the English High Court (Queen's Bench Division) wear, at sittings of the Court of Law, their scarlet robes (See court dress). Also in the United Kingdom, other civil dates have been added to the original religious dates. These include anniversaries of the Monarch's birthday, official birthday, accession and coronation.
The term "red letter day" is colloquially used to indicate any date of personal significance.
Oh, today is definitely a day of personal significance!
Our son Casey, his girlfriend Cassie and their TWO BIG DOGS moved out today!
They leased a house across town. No more sharing our small house with too many people and too many dogs. Now our dog and cat can roam free. And so can we!!!
I wish them many happy times in their new home. And also the same to Kim and I!