The Seville Fair is held in the Andalusian capital of Seville, Spain. The fair generally begins two weeks after the Semana Santa, or Easter Holy Week.

The fair officially begins at midnight on Saturday, and runs seven days, ending on the following Saturday. Each day the fiesta begins with the parade of carriages and riders, at midday, carrying Seville's leading citizens which make their way to the bullring, La Real Maestranza, where the bullfighters and breeders meet.

For the duration of the fair, the fairgrounds and a vast area on the far bank of the Guadalquivir River are totally covered in rows of casetas (individual decorated marquee tents which are temporarily built on the fairground). 

These casetas usually belong to prominent families of Seville, groups of friends, clubs, trade associations and political parties. 

From around nine at night until six or seven the following morning, at first in the streets and later only within each caseta, there are crowds partying and dancing sevillanas, drinking Sherry, manzanilla or rebujito, and eating tapas. 

The Fair dates back to 1846 when it was originally organized as a livestock fair by two councillors born in Northern Spain, Basque José María Ybarra and Catalan Narciso Bonaplata. Queen Isabel II agreed to the proposal, and on 18 April 1847 the first fair was held at the Prado de San Sebastian, on the outskirts of the city.

It took only one year before an air of festivity began to transform the fair, due mainly to the emergence of the first three casetas, belonging to the Duke and Duchess of Montpensier, the Town Hall, and the Casino of Seville. During the 1920s, the fair reached its peak and became the spectacle that it is today.

According to en.wikipedia