Q. A colleague wants to use a hyphen in the phrase “Friday-afternoon lecture.” But isn’t this an overly rigid application of the phrasal adjective hyphenation rule in a case where it doesn’t apply? “Friday afternoon” is not a true phrasal adjective, but a temporal phrase. “Join me for Sunday morning brunch” is the same as saying, “Join me for brunch (on) Sunday morning.” Interested in your view on which is correct, and why.
A. “Noun + noun” phrases like “Friday morning,” where the first noun modifies the second noun, do qualify as phrasal adjectives. A hyphen increases readability, since Friday followed by a noun is not always part of a phrasal adjective: a Friday golf outing; a Friday birthday party. See section 2 of CMOS 16, 7.85 (“noun + noun, single function”).