What is a "Red Letter Day"?

Another interesting bit of trivia from Eric "CalendarWallah" Snyder

http://twitter.com/calendarwallah
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clipped from en.wikipedia.org Red letter day A red letter day (sometimes hyphenated as red-letter day or called scarlet day in academia) 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.