Short answers, pulled from the story.
The Tumult of Aranjuez was an uprising against Prime Minister Manuel Godoy and Charles IV of Spain that took place on 17-the 19th of March 1808 in the town of Aranjuez, about 48 km south of Madrid. Soldiers, peasants, and members of the public seized Godoy, forced his dismissal, and compelled Charles IV to abdicate in favor of his son Ferdinand VII.
Godoy was resented by the nobility for rising to power from poverty and obscurity, and by the general public for aligning Spain with France against Britain, his conduct at court, and signing the Treaty of Fontainebleau, which allowed French troops to cross Spain and occupy cities including San Sebastián, Pamplona, and Barcelona.
Ferdinand and his supporters were central instigators of the uprising. He had already opposed Godoy by leading the El Escorial Conspiracy months before the March 1808 events, and his supporters joined disgruntled citizens in the assault on Godoy's quarters at Aranjuez.
In early May 1808, Napoleon invited both Charles IV and Ferdinand VII to Bayonne, France, under the pretense of resolving their conflict. Once there, he forced both to renounce the throne, then named his brother Joseph Bonaparte king of Spain in what became known as the Abdications of Bayonne.
The annual commemoration falls in the first week of September because when revived celebrations began in 1988 they were added on top of pre-existing September festivals already established in Aranjuez, rather than being placed on the historically accurate March dates.
The Treaty of Fontainebleau allowed French Emperor Napoleon's troops to cross Spain to attack Portugal. Spanish citizens experienced the resulting military occupation as a humiliating invasion, with French forces quickly seizing San Sebastián, Pamplona, and Barcelona, and they blamed Godoy for having agreed to these terms.