Every year around this time, a question of the utmost importance occurs to me.
I refer, of course, to the question of whether this holiday honors mothers or a mother—that is, whether the apostrophe should go before or after the S.
Fortunately, since the advent of Wikipedia, it's easy to answer that question definitively. The Wikipedia article on the topic quotes a Vancouver Sun article from 2008. The article is about Anna Jarvis, who trademarked the term "Mother's Day" in 1912; it notes:
She was specific about the location of the apostrophe; it was to be a singular possessive, for each family to honour their mother, not a plural possessive commemorating all mothers in the world.
Of course, one could go against Jarvis's wishes—it's not like we pay much attention to her other intentions for the day:
"I wanted it to be a day of sentiment, not profit," Jarvis complained, dismissing greeting cards as "a poor excuse for the letter you are too lazy to write."
But other sources also suggest the singular apostrophe placement. For example, from Wikipedia again:
[The singular apostrophe has also been] used by U.S. President Woodrow Wilson in the law making official the holiday in the U.S., by the U.S. Congress on bills, and by other U.S. presidents on their declarations.
And MW11 supports that punctuation as well. So I'll go along with it.
(Yes, I could have just checked the dictionary in the first place. But this route was more interesting. I had no idea the term was trademarked, for example.)
