Definition of "red-letter" [red•-let•ter]

• Memorably happy: a red-letter day. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "A lot of people here identified as red-letter Christians, who believe the words of Jesus have been hijacked by worldly authorities."
• "Given its roots in fox hunting, which greeted each 26 December as a red-letter day, national hunt racing's King George VI Chase at Kempton is a Boxing Day staple since 1947 that has provided imperishable memories of courage and drama."
• "On one red-letter day he sent 19 witches to their deaths."