Why are the shops closed in Barcelona on Monday?

My first task on Monday was going to be tackling the Vodafone shop to try to get a Spanish sim card. Ten oćlock no-one in sight, walked around the block, not the worst block in world to walk around, 10.30 nothing doing, 11am nothing else open. Walked several more blocks to another Vodafone shop. Closed. Gave up and took the metro and fenicular to Montjuic. Montjuic is a castle on a hill overlooking the city, port and was where most of the Olympic events were held, including the spectacular diving platform with the Segrada Familia as the backdrop. Unfortunately it has also been the venue of many executions, most notably the former President of Catalonia in 1941 after he was dobbed in by the nazis in France to Franco. It also features prominently in Carlos Ruiz Zafronś book The Shadow of the Wind. The view is spectacular, but I couldn't help feeling a bit creepy, there was a feeling bad things have happened here. Tried again at the Vodafone shop, but not even open at 8pm. Gave up, but got to see a lot of Barcelona.