I have this theory about why so many balls show up in fantasy novels. Part of it is because the scene they create is so pretty.
But the bigger part of my theory hinges on prom. No, really. I had a totally craptastic prom experience. (Junior and senior. I don't know which was worse, the whiny boyfriend from junior year who got mad at me because I didn't tell him he looked nice every five minutes, or the chainsmoking date from senior year who wore SHORTS with those socks that come halfway up your lower leg.)
So getting to read or write about a ball is actually a fantasy prom that none of us ever got to go to. No, really. I'm serious!
My favorite kind of ball, and my absolute favorite fantasy trope that I can NEVER get enough of and probably will never ever get tired of is the masquerade. I think they're gorgeous to read (and to watch!) and there are so many possibilities during a masquerade that don't exist anywhere else--not even in other balls. Come as you aren't, isn't that the theme? Wear a mask and become something or someone else for the night.
Masquerades are full of possibilities. Mistaken identity. Love without ever seeing someone's face. Disguising a prince or an assassin or a milkmaid. (Disguising your smallpox scars or the fact that your nose fell off because you have syphilis. Hah.) And, of course, not using a disguise at all, but instead choosing a mask on a stick that can be removed and that doesn't hide your face very well to begin with.
If you want to make sure your masquerade is unique, the biggest bit of advice I can give (and this comes from reading tons and tons of fantasy novels) is to make sure there's a reason to have the ball. Not necessarily that the characters throwing the masquerade have to have a good reason (people do stuff for fun all the time), but make sure that the plot needs the masquerade--and specifically a masquerade, rather than just any old ball. Otherwise they can seem like gratuitous scenes and might end up darlings that need to be murdered. (At the same time...I sort of don't mind even if they are gratuitous.)
An example: the story of Cinderella always has a ball, because that's where the prince meets her and where she loses her glass slipper etc. etc. etc. But in ELLA ENCHANTED by Gail Carson Levine, she changed the ball to a masquerade, because the prince and Ella had already met, and he couldn't know her identity at the ball--so having a masquerade ball gave Ella the opportunity to wear a mask. She made it unique and more useful to the plot. (Plus it's a brilliant book.)
Masquerade balls and the reasons for having them can say a lot about your country/society's economy, and people's reaction to it (especially people in power). Are they frivolous and very Marie Antoinette-ish? Spending tons of money when their country can't afford to eat? Or are they really in an economic boom and everyone is happy? Or, more likely, perhaps the country is in war-time and people need a pick-me-up. Or the aristocracy is trying to pretend like nothing's wrong when really the treasury is empty and everyone is secretly broke. Another one of my favorites is some kind of show of riches or power for a neighboring nation who might be visiting at the time. I'm not so much a fan of the idea of throwing a ball to find a mate for a prince or princess, since that one gets used more often, but those balls do have their place in the plots of fairytale retellings.
Do you have a favorite masquerade (or ball) scene from a book or movie? (I feel like the obvious answer is PHANTOM OF THE OPERA...)