Hi!
Reading between the lines I suspect your question is whether riots or violence is likely, right? The short answer is NO... the day is simply taken just as another bank holiday. There aren't any demonstrations or any other civil acts. Many of us simply feel is sort of a 'foreign' holiday, if you catch my drift, so there isn't any reason to celebrate anything. Not even in our National Day (Sept 11th), when millions get out to celebrate (and protest too) there is any worth mentioning incident, we approach this issue differently here. I've already published this link many times before, but should you have missed it, this is a short video of one of our last Sept 11th in Barcelona, with over 2 million in the streets: http://infocatalonia.eu/w/6pNdr
As per longer holiday, well, let's first explain to other readers that this is an odd (stupid?) situation: December 6th is Spanish Constitution Day and 8th is a religious festivity related to Virgin Mary (Immaculada Concepció, festivitat de la Puríssima). So as in years like 2016, where 6th is a Tuesday and 8th is a Thursday, there is a surrealist situation in which one goes to work on Monday, Wednesday and Friday only... totally unproductive and simply nuts! For long it has been discussed that this should be fixed and one of them to be "moved" to another day but neither the Spanish government nor the Catholic Church wants to give in. So what happens is that some companies/shops take the Monday as a bank holiday while others take Friday instead.
Yet if the question is "how does it affect me as a tourist?"... the answer is "it doesn't" -at least in Barcelona and most of Catalonia- since most attractions, parks, museums and other events that might interest you will be open, so will be the restaurants and some convenience stores. Needless to say, transportation is not altered either (actually it never is any day of the year except a bit maybe on Dec 25th and Jan 1st). Still, do check the websites of the specific activities you were going to be doing that day to make sure they are not "the exception" to the rule. Also, don't plan on "shopping" those days.
Enjoy!
PS: Obviously whether peaceful demonstrations will continue to be, as it has been a long tradition in Catalonia, will depend entirely on the Spanish response to the quest of our government for independence. As it has appeared several times in the media these past 5 or 6 years, some nuts in their military, as well as some hyperventilated Spanish politicians, have called for the use of violence to quench the Catalan peaceful aspirations based on democratic legitimacy. Should that happen it would then of course be a completely different ball game.