We're currently planning on being in France or Andorra on September 11, but could possibly reorder our itinerary to be in Barcelona on the National Day of Catalonia. Would this be advantageous or disadvantageous? I imagine there might be interesting activities going on and it might be interesting to watch the locals mark the occasion, but if it comes at the expense of major sights being closed or dealing with large crowds, no thanks. Our itinerary is fairly tight as it is.
... it's a unique experience, happening only once a year and worth seeing.
Besides, what 'sites' would you be visiting?... Barcelona has over 150 sites worth visiting and many, especially the most popular, remain open for most of the year (ie. La Pedrera, Casa Batlló, Park Güell, Sagrada Família...)
As per dealing with large crowds, that's something unavoidable on such a day, especially in certain areas of the city.
Totally agree with "Wonderful" (see post above) about Andorra. If it is not ski season there is nothing to do but shop. The drive up was fun, though.