I will be in Madrid in August and need to purchase a guitar case there. I will be attending a workshop in classical guitar construction and need the case to bring the guitar I will be building back to the US.
I have read in many posts on MM's forums that lots of businesses 'close up' in August for vacations. I'm wondering if there will be any music or other stores open where I can purchase a guitar case?

Big department stores will be open in August, but most smaller commerces close. There is 1/3 or 1/4 probability that your favourite guitr shop wll be closed. Fortunately, there are several specialized shops, so you'll probably find one that's open.