Definition of red-letter day in English:

red-letter day

Syllabification: red-let·ter day

noun

A day that is pleasantly noteworthy or memorable.
More example sentences
  • Because feast days in such almanacs and calendars were frequently written or printed in red, a red-letter day came to be a term for one that was special.
  • Still, pub owners say, it was a red-letter day for business and they are planning a second block party for the first weekend of November.
  • Today, as I said, is a blue-ribbon day, a red-letter day, or whatever one wants to call it.

Origin

early 18th century: from the practice of highlighting a festival in red on a calendar.

Definition of red-letter day in:

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Word of the day anomalous
Pronunciation: əˈnämələs
adjective
deviating from what is standard, normal, or expected