Subbing for Santa

In your world Santa Claus, The Grim Reaper, the Tooth Fairy, any popular mythical figure actually exists but either something happens which causes them not to be able to actually do their magical duty. Perhaps they fall sick, possibly they've been kidnapped, maybe they've died in an accident or perhaps the legend wishes to teach someone a lesson in humility. Maybe it's a case of Someone Has to Do It and there is only one who can step in when Death Takes a Holiday. Who'll step in? You, of course! Subbing for Santa is when the protagonist of a story takes the mantle of a mythical being and takes over their job, even if it's just temporary.

This is not when a character is infected by a creature and so becomes that creature, like with some vampires or some werewolves. The mythical task has to be explicitly given or falls upon the protagonists because of the circumstances of them maybe being in the wrong place at the wrong time or maybe because they're the only one who can at that moment.

In the event of an inversion, a character will steal the mantle of the mythical figure in order to reap the benefits or the glory for their own. In such cases, the main characters may need to save Christmas or lives, or the world even in order to set things right.

Compare: You Kill It, You Bought It, Someone Has to Do It, God for a Day, Saving Christmas, Death Takes a Holiday, and may lead to An Asskicking Christmas.

Comics

 * An issue of the Looney Tunes comic book featured a story where Bugs Bunny and Daffy Duck substitute for Santa Claus and one of his reindeer after the latter injures them.
 * The Grim Reaper is replaced by an impertinent little upstart in issue #9 of the Bill and Ted comic.

Film

 * In Fred Claus Fred temporarily taks his brother's mantle.
 * In Bruce Almighty Jim Carrey's character is given the powers of the Christian God in order to teach him a lesson in humility. And in its sequel Evan Almighty Steve Carrell's character is bestowed the fantastic non-powers of Noah.
 * The Santa Clause movie and its follow-up films:
 * In the first film, the Santa Sub is Scott Calvin for, well, Santa Claus.
 * In the first sequel the Santa Sub is Principal Newman in that she must become the new Mrs. Claus, through a Mrs. Clause. binding magical contract.
 * The third film has an inversion in that Jack Frost becomes Santa by tricking Scott Calvin into giving up the role.
 * An inversion occurs with The Nightmare Before Christmas: The King of Halloween, Jack, wishes to take over the job for Santa, so has him kidnapped. Despite his good intentions, Jack forcefully takes Santa's mantle and the entire Holiday.
 * The Tooth Fairy film with Dwayne Johnson.
 * Like the above movie, Toothless has Kirstie Alley's character take up the mantle of Tooth Fairy when she goes to Limbo. Other beings you have to become in Limbo include Santa Claus, Easter Bunny, Cupid, Cinderella, a Boogeyman, and Sleeping Beauty.
 * Pirates of the Caribbean:
 * Santa Baby: Santa's daughter takes the reign.

Literature and Folklore

 * This is a staple of the Death-centric Discworld books.
 * In Mort, Death takes an apprentice who has to substitute for him when he wanders off to go soul-searching.
 * In Soul Music, Death goes missing, and Susan has to temporarily take his place.
 * In Hogfather, the Hogfather goes missing on Hogswatchnight, and Death has to temporarily take his place.
 * Another example from Hogfather - when Bilious was embarrassed to admit to being the Oh God of Hangovers he made up a story about how he does this on a professional basis, substituting for any deity that needs time off.
 * Sometimes in Greek Mythology tragic hero or demi-god like Heracles would temporarily take the mantle of another god. Like Hercules being tricked into holding up the sky by Atlas or Phaeton taking the chariot reigns from his father, Helios.
 * In the Percy Jackson and The Olympians book The Titan's Curse,  so he can lead the Titan army..

Live Action TV

 * One or two episodes of Sabrina the Teenage Witch had the titular character delivering presents at Christmastime. And one time Sabrina takes a local Sandman duty as a summer job.
 * Saturday Night Live: "The Night Hanukkah Harry Saved Christmas." Santa is sick so Hanukkah Harry steps in to help out. In a sequel he and the Prophet Elijah step in for the Easter Bunny after the bunny gets caught in a steel trap.
 * In Charmed when Piper became the Angel of Death.
 * Paige was once turned into a nymph to replace one that just died.
 * Charmed also had many inversions of demons and warlocks and other bad guys stealing the powers of good magical creatures for their own gain.
 * Just to name a few:
 * A Demon steals Cupid's ring and starts breaking up relationships in revenge.
 * A warlock steals a ring which allowed him to harness the powers of Muses.
 * A warlock stole the powers of a Whitelighter in order to get into heaven and kill all the elders.
 * A demon once used a Sandman's dust in order to bring the Charmed One's dreams to life.
 * Dean became Death in an episode of Supernatural, as part of a deal. He finds he can't go through with seeing all the people dying.
 * The Twilight Zone's The Night of the Meek deals with a santa example.

Music

 * Played with in the Star Wars Concept Album, Christmas in the Stars. C-3PO and R2-D2 anxiously await Santa Claus's coming, only to be shocked when the one who actually delivers their presents turns out to be his son, "S. Claus". S. Claus explains to them that Santa couldn't possibly deliver all the gifts in the galaxy, so he has his family assist in delivering them.

Mythology

 * While Atlas was never indisposed, Hercules needed a god to pick the apples of the Hesperides because as part mortal he couldn't. So Herc offered to switch places temporarily with Atlas. Herc had to think quickly when Atlas almost bailed on him.

Western Animation

 * Several Christmas specials have the protagonist having to fill-in for an indisposed or sick St. Nick.
 * The Pinky and The Brain Christmas special has Brain stealing the role of Santa to distribute a mind control device to every child on the planet.
 * On Family Guy Peter takes over as Death when he twists his ankle.
 * And on another episode of Family Guy Brian and Stewie sub for Santa when they discover that all the deliveries are killing him. Hilarity Ensues... to a point. The events that play out can be viewed as a Deconstruction of this trope, if not the entire concept of Santa Claus in general.
 * In an episode of The Powerpuff Girls, the titular trio try to stop the villian Princess Morebucks from taking everyone's presents Santa delivered, and thus ruining Christmas. They stop her later, but damage is already done, and there isn't enough time for Santa to deliver the presents by morning. The solution? Have the super fast Powerpuff Girls deliver them!
 * An episode of Johnny Bravo had Johnny having to deliver the gifts after he attacked and beat up Santa mistaking him for a burglar.
 * A Flintstones Christmas special (Yes, prehistoric cavemen celebrate Christmas) centered around Fred filling in for Santa with Barney helping as an elf. In this case, Santa has a leg injury and has to stay at Fred's house, but maintains radio contact with the sleigh when the two need advice.
 * In an actual Flintstones show episode, Fred's been working as a Santa's helper at Macyrock's department store. He takes a nap at quitting time and is waked by elves who ask him to take Santa's place while Santa has a bad cold. (This would seem to indicate it was All Just a Dream but the rest of the episode is played straight.) At the end Fred realizes he hasn't any gifts left for his own family, only to hear sleigh bells overhead and "Merry Christmas, Fred! Aaaa-choo!"
 * In the Jackie Chan Adventures Christmas Episode, Santa is kidnapped by the Big Bad and Tohru has to fill in for the night. Jade and Paco join him as elves, keeping them out of trouble, for once. Played straight again in the third season of the series. When the 12 Talismans of the Chinese Zodiac are destroyed, the powers fall onto one each of the 11 actual animals (Dragon float in limbo because Komodo dragons apparently don't count).
 * Phineas and Ferb: Due to Doofenshmirtz' Naughtyinator, Santa won't be visiting Danville to deliver gifts, so Phineas, Ferb, and company decide to deliver the presents themselves.
 * Subverted at the end.
 * Olive the Other Reindeer: A variation. Olive takes over for one of the reindeer after he breaks his leg.
 * On one of The Simpsons's Treehouse of Horror shorts, Homer takes Death's mantle after death has a mishap.
 * Disney's Hercules did this quite a few times where Herc had to sub for quite a few gods, such as driving Apollo's chariot, or covering the land with Morpheus' blanket of sleep.
 * More an example of Switching with Santa, in one episode of The Grim Adventures of Billy and Mandy, Grim and Billy switch lives for a day leading to Billy taking over as The Grim Reaper and, well, Hilarity Ensues
 * The Grim Reaper being a job in TGAOBAM, other people can take that job. In the Movie, Grim is fired, and is replaced by Numbah Three, before he gets his job back.
 * In the Futurama episode "A Tale Of Two Santas", Bender has to take the place of Robot Santa.
 * Merry Madagascar: The cast of Madagascar has to deliver the presents after Santa comes down with a case of Easy Amnesia.
 * In the Looney Tunes short "Easter Yeggs", it turns out the Easter Bunny always gets some poor schlub to do his deliveries for him, and this year's schlub is Bugs Bunny.
 * In Sonic Christmas Blast, both Sonic AND Robotnik pulls this off - Robotnik forcing the kids to give him toys and Sonic rescuing, then being promoted to the Santa of Mobius!