First celebrated in 1886, it's the national day of Catalunya with a public holiday making this a long weekend. There are various events and celebrations throughout the day, beginning with the commemorative laying of wreaths by various dignitaries, including past and present presidents of Catalunya as well as by the Mayor of Barcelona, at the Rafael Casanova monument. Throughout the day there will be concerts at the Plaça de Catalunya, whilst a number of government buildings, such as the Catalan parliament, will hold an open house day for the general public. More information here.
For their annual demonstration, this year L'Assemblea Nacional Catalana (ANC) will fill the Avinguda Meridiana with a white mosaic, later to be filled with colour. See here for more.