I’ve always loved explaining to my students that the Spanish word for vacation, vacaciones, is almost always plural. “Spanish people love vacations so much that they never take just one!” This is one of those fun linguistic facts that appear to be culturally indicative regardless of their actual history.
In fact, if you look up the abbreviation U. m. en pl. (“Usado o usada más en plural”) in the Real Academia’s own documentation you will see that vacaciones is one of 664 normally plural words in Spanish, and they aren’t a particularly fun group. They include accesorias (added-on wings of a building), alineamientos (“alignments”), and alpes (a tall mountain, obviously derived from the Alps). You can see that I didn’t get beyond the a‘s…
I don’t know how vacaciones developed its standard plural usage. It isn’t universal in Romance. The French and Portuguese words are also plural (les vacances and férias, which obviously uses a different Latin root). But Italians and Romanians take singular vacations (vacanza, vacanţă), which implies that the plural usage is a Western Romance innovation.
Regardless of the history and linguistic context of vacaciones, I was delighted when I happened to look up the etymology of fiesta today and saw that it comes from the Latin plural festa (the singular is festum). Of course the etymology has no relevance for how people think of the word today, even if they know its origin. But it’s a delightful coincidence nevertheless. You have to love a language where vacations and parties are intrinsically plural!