Sue Gold, Communications Director of Westtown School, was one of two Good Word readers who asked the question: “Why do you have to put an apostrophe before the s in la-di-da’s?”
Good question. The traditional answer is that since “la-di-da” is not a real noun or verb, the apostrophe is appropriate. Words and other things used as major lexical categories have traditionally been marked by using an apostrophe between them and any suffixes that accrue to them, especially if omitting the apostrophe results in a odd-looking form.
Many writers in the US are moving away from this rule, though. I’ve long since given up on writing the decades with apostrophes, e.g. 1980s rather than the traditional 1980′s, since it is a number, not a noun.
In the midst of change like this, when there is no basis for a choice, I sometimes make my choice democratically just to keep the decision from being totally arbitrary: the la-di-da’s outnumber the la-di-das 2 : 1 on the Web (today). This fact probably reflects the fact that the non-noun rule is still in practice in all the other English-speaking regions of the world. Of course, democracy is not the way matters of style are settled so the question remains an unsettled one.
Of course, you can also use an H in this case: la-di-dahs.