Royal Request Day is an event held by the Rainbow King (presumably annually) to grant special requests of the Rainbow Kingdom citizens. Hundreds of citizens across the kingdom form a line leading to the Rainbow Castle's throne in hopes that their request will be granted. It is unconfirmed if the Rainbow King grants every request, but he has granted requests for the sky to be purple, pink, and green (respectively) among others.
The event is only highlighted in "A Royal Stink". In the episode, due to the Rainbow King's cousin Glummy Glooma causing negativity across the kingdom, the Rainbow King leaves True and Bartleby in charge of granting the requests, and names True "queen for a day".