“7.20 “For . . . sake” expressions
For the sake of euphony, a few for . . . sake expressions used with a singular noun that ends in an s end in an apostrophe alone, omitting the additional s.
Euphony, huh? Where else in English is euphony valued highly enough to throw a basic rule under the bus? Chicago is steadfast about using an “s” after the apostrophe in all possessives, except here.
It seems like this is a nod to spoken English, where the sound of words is much harder to ignore. Sound can’t be ignored completely in writing, but I can’t see why “for goodness’ sake” would be allowed except to make it easier to say and hear. Especially since it’s spoken exactly like “goodness”. Grammatically, it sounds wretched. But it’s here to stay–no one wants to re-learn their Christmas songs.