Beginning this weekend is Santander's annual fiesta de Santiago, leading up to Santiago, Jul 25. There are daily bullfights so you should be able to see the one you want.
Continental-Auto has express Bus Service from Madrid's Avda. de America Intercambiador. Some are nonstop to Torrelavega, and the total time it takes is 5.5 hours. It's a lot cheaper than the train. As you come out of the Bus Station, there are usually several people offering inexpensive lodging.
I love Santander, especially playa de Sardinero. The Calle de Burgos really rocks when the sun goes down. Try Taberna Narda, C/ Captain Palacios, 3, for a great seafood luncheon menu. It's where the Truckers eat.