Who’s Mindful of Who’s Apostrophes’?

While rambling incognito through a remote town, I saw this sign gracing the upper floor of a restaurant: “The Party’s at Sally’s.” I was intrigued. The quotes look out of place, I thought, unless the whole is the title of a movie or musical. However, the smell coming out of the building in conjunction with rather wild music convinced me that the establishment was indeed an “eatery” in which the party was never over. But forget the quotes. What is “party’s”?
Who’s Mindful of Who’s Apostrophes’?