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.
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The First Council of Nicaea in 325 decreed the saints' days, feasts and other holy days, which came to be printed on church calendars in red.
Red Army | International Red Cross and Red Crescent Movement | American Red Cross | Red Hot Chili Peppers | The Church of Jesus Christ of Latter-day Saints | Green Day | Red Sea | One Day International | New Year's Day | Boxing Day | Memorial Day | Seventh-day Adventist Church | Labor Day | Red Star Belgrade | Red | Doris Day | Little Red Riding Hood | Detroit Red Wings | Earth Day | Red Bull | May Day | Red River | Red Hat | Sandra Day O'Connor | Red Dwarf | Valentine's Day | International Committee of the Red Cross | Red River of the North | Record Store Day | New York Red Bulls |
Dr. Judith Mossman (voiced by Michelle Forbes) first appears briefly in Red Letter Day, in which she is attempting to stabilize Gordon Freeman's damaged teleportation.