On Sundays most of the people go to La Latina (the old quarter of Madrid) to have tapas on any of the wine and tapas bars that litter the streets. Some of the best restaurants of Madrid are also in this area: Lucio, Casa Paco, Botín, El Cosaco, el Alamillo, etc. some of them are closed in August.

Andrés
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Tapas events in Old Madrid :
http://www.madrid-tapas-parade.com