Definition of red-letter in US English:

red-letter

verb

  • with object To mark out (a person or thing) for particular attention with, or as with, red letters.

Origin

Late 18th century; earliest use found in John Gutch (1746–1831), antiquary and Church of England clergyman. From red letter.

Pronunciation

red-letter

/ˌrɛdˈlɛtə/