Is there a historical basis for the "day of the false King" described in The Egyptian?

Question

In Book 6 of Mika Waltari's historical novel The Egyptian, Kaptah, Sinuhe's slave and companion, ascends to the throne of Babylon for a day:

I was by now familiar with many customs in Babylon, yet I was astonished to see the King’s bodyguard before dawn on that day crowding drunkenly into Ishtar’s House of Joy, breaking open the doors and striking everyone they met with the butts of their spears as they shouted at the tops of their voices:

“Where is our king hiding? Bring him forth speedily, for the sun is about to rise, and the king must dispense justice to his people!”

...

But when they had had enough of mockery, they released me, saying, “Waste no more of our time but deliver to us your servant, for we must bring him to the palace with all speed, today being the Day of the False King. It is the King’s will that we hasten to the palace with the man.”

When Kaptah heard this, he was so terrified that he began to tremble and shake the whole bed so that they found him and, dragging him forth with jubilation, made deep obeisance before him.

They said to one another, “This is a day of great rejoicing, for at last we have found our king who had hidden himself and disappeared from our sight. Our eyes are gladdened by him, and we hope he will reward our fidelity with many gifts.”

_{Waltari, M., & Walford, N. (2002). The Egyptian: A novel. Chicago: Chicago Review Press.}

Wikipedia claims the event has "its basis on reality", and points to Rajala's Unio mystica: Mika Waltarin elämä ja teokset for evidence. Unfortunately, I couldn't find a translation and cannot verify the claim.

Help?

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The "day of the False King" appears to be central in Brad Geagley's novel Day of the False King, and the novel's Goodreads description also claims that it has a historical basis:

As in "Year of the Hyenas," most of the events and characters in "Day of the False King" are drawn from history. The Elamite invader King Kutir and the native-born Marduk truly vied for the throne of Babylonia. There really was a festival called "Day of the False King," when the entire world turned upside down for a day, when slaves ruled as masters, when the most foolish man in Babylon was chosen to become king. Semerket the detective is plunged into the midst of these events in pursuit of his own goals: to serve his Pharaoh and to find the woman he loves.

Answer 1

I'm the editor who had added the info to Wikipedia. I heavily expanded the novel's article in 2018, but had maybe gotten a touch lazy when I began tackling the section about historical accuracy, with terse explanations; I had suddenly become busy with real life, and soon returned my sources to the library. I have now added to the bit about the day of the false king according to the source (Unio Mystica), briefly describing the tradition.

Here's the source's passage in question:

Suurin osa Waltarin kuvaamista uskonnollisista menoista ja tavoista pitää paikkansa, Holthoer todistaa. Väärän kuninkaan päivä Babylonissa vaikuttaa fabuloivan fantasian tuotteelta, mutta silläkin on tarkka taustansa. Jos ennustukset kertoivat kuninkaan kuolemasta, oli pestattava sijaiskuningas päiväksi ja surmattava hänet, jotta ennustus kävisi toteen, mutta ei vahingottaisi oikeaa kuingasta. Tapa tunnettiin sekä Babylonissa että Egyptissä. Kirjallisena aiheena moukan istuttamista hallitsijaksi on käytetty aina Tuhannen ja yhden yön tarinoista Ludvig Holbergin Jeppe Niilonpoikaan saakka.

My translation:

The majority of religious ceremonies and practices portrayed by Waltari are correct, Holthoer attests. The day of the false king in Babylon seems like the product of a fabling fantasy, but it too has its exact background. If prophecies foretold of the king's death, a substitute king had to be appointed for a day and be killed, so that the prophecy would be fulfilled, but wouldn't harm the real king. The custom was known both in Babylon and Egypt. As a literary subject, the seating of a boor as a ruler has been used all the way from One Thousand and One Nights to Ludvig Holberg's Jeppe on the Hill.

For future reference, you can add {{Verification needed}} or even {{Request quote}} to a Wikipedia statement in situations like this.