La Merce Festival

Here ya go, from the official city website (in English): https://www.barcelona.cat/lamerce/en

You can search by location and activity type, some specific events I would recommend:

Passejada de dracs i bèsties de foc - El Born

Falcons de Barcelona i Falcons de Vilanova - Plaça de Sant Jaume

Cavalcada de la Mercè - Ciutat Vella

Ballada de sardanes - Plaça de la Mercè

Concert extraordinari de les festes de la Mercè - Palau de la Generalitat

PS - La Mercè will be a field day for pickpockets, so be extra weary, but still have fun!

To add to what Carlos said, your hotel will probably have printed programs (ours did) you can take with you. When watching the Castellers, the Plaza was literally packed with people, shoulder to shoulder to the point it was hard to move, or exit the plaza. The festival and events were fantastic. To re emphasize, watch your belongings closely.

Edit to add: I could not find the specific event for the Castellers, the Human Towers so emblematic of Catalonia. But this is an event you should definitely look out for, they are usually at the Plaça Sant Jaume, but you can ask your hotel for more details.