“La Mercè (Catalan pronunciation: [ɫə mərˈsɛ]) is the annual festival (festa major in Catalan) of the city of Barcelona in Catalonia, Spain. It has been an official city holiday since 1871, when the local government first organized a program of special activities to observe the Roman Catholic feast day of Our Lady of Mercy, La Mare de Déu de la Mercè in Catalan. Although the actual feast day is September 24, the festivities begin a few days beforehand.
Some of the most important features of the festival were introduced in the year 1902, when parades included papier maché “giants” known as gegants i capgrossos, the first Castell competition in the city, and a popular dance from Empordà that was becoming popular throughout Catalonia: the Sardana. The holiday has enjoyed immense local popularity ever since.
Among more recently introduced traditions are the annual Catalan Wine Fair, a special correfoc, a 10 km race and the pyro-musical, a display featuring synchronized fireworks, water fountains and music conducted at the base of the Montjuïc mountain.
The celebration of la Mercè has religious origins, honoring the Virgin of Grace (Mare de Déu de la Mercè), patron saint of the archdiocese of Barcelona, and co-patroness—along with Saint Eulàlia—of the city. In Catalan, the word mercè has meanings related to service, help, a sense of compassion, and loving mercy. In the Gothic Quarter of Barcelona, there is a basilica dedicated to the Virgin, where a wooden image of her is venerated.
The festival has been celebrated since the Middle Ages when, in 1687, Barcelona suffered a plague of locusts. To fight against insects, the Consell de Cent, which then governed the city, voted to ask the Virgin’s assistance. When the city was delivered from the pestilence, she was named patroness of the city of Barcelona— although this was only recognized by the Pope in 1868. Since that time, an annual festival has been celebrated in the city in honor of the Virgin, starting on the 21st of September and finishing on the 25th.” www.wikepedia.com