A day marked in red on calendars; a church feast day. / (idiomatic) A particularly significant day; a day of personal or sectarian celebration.

The postal facility that deals with mail that cannot be delivered.

A reference to any of several of the strongest English swear words that are also four letters long, especially those composing the so-called seven dirty words.

plural of red-letter day

alternative form of red letter day

A short, hand-written communication to thank someone who has recently provided the writer with hospitality, usually dinner or an overnight visit.

(law) The law as a set of specific concrete rules set down by longstanding precedents.

plural of four-letter word

A law that is a large-scale attempt by a government to regulate business in the interest of society at large.

plural of Dear John letter