Princess for a Day

This is whenever a low- to middle-class girl gets to put on some fancy clothes and pretends to be a member of the upper class for a short time—maybe even a princess. Often it's to go to a royal ball, but it could be for different reasons.

It could be part of a Massive Multiplayer Scam, or because she's been telling a loved one or rival living somewhere else that she is part of the upper class. If it's the latter, expect the deception to be exposed, and An Aesop about honesty and being yourself. Other times, it's just that girl showing up and having a good time.

In many of these plots, one other person happens to know the true identity of this unfamiliar lady, but can't expose it without bringing down scandal upon himself.

Despite the title, this happens with guys as well.

Often involves She Cleans Up Nicely and Pimped-Out Dress. Also a key component of a Prince and Pauper plot.

A Sub-Trope of Penny Among Diamonds.

Compare Mock Millionaire.

Examples of Princess for a Day include:

Anime And Manga

Dawn in "Dawn of a Royal Day". She swaps places with Princess Salvia so the latter can participate in a Pokemon Contest. Salvia enjoys herself, Dawn doesn't. She gets a Togekiss out of the deal, though.
The girls of Sailor Moon do this when they go to the princess finishing school for one episode.
- There's also an early chapter in the manga, when Sailor Moon investigates a party being held in honor of Princess D. She uses a magic pen to morph into a princess, and has fun until the enemy attacks. She also gets a dance and a kiss with Tuxedo Mask (although she doesn't seem to remember the latter, seeing as she was drunk at the time)
Makoto from El-Hazard: The Magnificent World is forced to replace Princess Fatoria until such a time that she can be found and rescued. Makoto, a guy, isn't fond of this idea.
It's not a party, but at one point in Fruits Basket, Tohru is invited to take a vacation at a Sohma family-owned spa, as a Golden Week gift. Lampshaded when a blown-over Tohru says that she feels like a princess.

Comic Books

Faith in Buffy the Vampire Slayer Season 8, No Future For You.
In an early Batman story (almost certainly inspired by the Lady for a Day movie), Batman persuades all of Gotham City to play along with a charade to allow a small-town girl to convince her parents that she is a successful actress.

Fairy Tales

"Cinderella"

Film

Miss Pettigrew Lives for a Day
Maid in Manhattan
The "glass slipper" of Ever After belongs to a peasant who pretended to be a courtier to save a man's life.
Jack dresses up like a first class passenger in Titanic.

Literature

Named for the Damon Runyon short story Lady for a Day, made into a film of the same name by Frank Capra, and then later remade by him as Pocketful of Miracles. In it, a woman called "Apple Annie" has been telling her daughter in Spain that she is a New York socialite. When the daughter arrives with her fiancé, the son of a Spanish count, a mobster who considers her apples good luck helps Annie maintain the charade to avoid humiliating her daughter and ruining the engagement.
In the Warhammer 40,000 novel Scourge the Heretic, the low-born, religious fanatic assassin Keira has to pretend to be an intellectual, free-thinking noblewoman in order to infiltrate a high-society Chaos cult. She pulls it off remarkably well.
Fanny Price of Mansfield Park, who has been The Unfavorite in her uncle's house her entire life, finally gets her coming-out ball (the second ball she's ever attended in her life) where she is treated with the respect and attention due a lady.
- Fanny is ignored previously because, until the ball, she is a child, not a lady, and should be seen but not heard.
Meg in Little Women gets this treatment in the chapter "Vanity Fair", and attends a high class party only to be humbled by ladies whispering behind her back about her and her family's poverty. That and those dancing slippers really hurt.
The Hawk And Fisher story, Vengeance for a Lonely Man, by Simon R. Green sees the main characters going undercover as minor nobility to catch a spy who's stolen extremely sensitive information. Interestingly a popular fan theory at the time had it that Hawk & Fisher were nobility - namely Prince Rupert & Princess Julia from the recently published Blue Moon Rising. The later Blue Moon Rising officialy confirmed this theory.
In Catherine, The Great Journey of The Royal Diaries she has nothing nice enough to wear to the royal court until the king lets her borrow a gown belonging to the king's sister. When she has to return it it reminds her of the story about the little ash girl who was beautiful until the stroke of midnight.
Vin does this several times in Mistborn, impersonating a noblewoman in order to gather information about what the nobles are planning and to try and pit different noble houses against each other.
There are a few times when Anne Shirley dresses up and goes on outings into town. One memorable occasion has her attend a charity event where she and a number of other people recite monologues. While she initially feels miserable, plain, and nervous amongst the famous people who are clearly much higher-class and wearing more expensive clothes than her, she impresses everyone with her monologue, is invited to meet and mingle with said higher-class guests, and later learns that a skilled painter had admired her hair.

Live Action TV

Kaylee in the Firefly episode "Shindig". Mal was partly this. At first, he was only interested in getting the job done, but then when he saw Inara was there, he decided to join in.
- Kaylee was an example of "just that girl showing up and having a good time"; she enjoyed herself tremendously except for one run-in with the Alpha Bitch (who got put in her place anyway). She wore a fancy dress but never pretended to be someone beside herself and this made her the most popular young woman there. All the young men (and a fair number of the not-so-young men) at the party were fascinated by a pretty woman who knew more about engines than they did.
A 1950s TV game show was Queen for a Day.
To an extent, Liz in the Thirty Rock episode "Black Tie". She got her fancy dress for the evening from the wardrobe department of the Show Within a Show.
Occurred in Gilmore Girls when Emily wanted Rory to attend a debutante ball. This was sort of a sore point for Lorelai, who didn't want Rory to have the same silver spoon upbringing that she did.
- Also in Rory's freshman year at college her grandmother wanted to make her into a princess to introduce Rory to Emily's friends' sons.
In Merlin, Gwen impersonated Morgana in one season 2 ep, after she and Morgana were captured by bandits. Morgana escaped and the henchman taking Gwen back begged her to impersonate Morgana in fear that the leader, Hengist, would kill him if he didn't bring back Morgana

.

Real Life

Truth in Television: Ladies' Day at the Ascot race meeting. To an extent. Guests were reminded in 2009 that they were required to wear underwear.

Theater

Pygmalion / My Fair Lady.
Adele in Die Fledermaus, the lady's maid putting on airs.

Video Games

In Infinite Space, in order to get somebody from the LMC who is willing to hear about Lugovalos threat, Nia disguises herself as a representative from Lanco Warship Manufacturing (using the dress that Yuri bought for her) to attend the conference at Zefiro. And a few hours later into the game, we learn that she WAS a princess...

Western Animation

Whopper did this in an episode of Pound Puppies.
In Avatar: The Last Airbender, Katara does this (with Toph's help) to infiltrate the Earth King's palace. Aang and Sokka try it, but it didn't work out as well for them.
Tiana in The Princess and the Frog, accidentally, which leads to the film's big twist.