Barcelona tickets

I'm taking a Europe trip at the end of the summer. Going to be arriving in Barcelona the morning of Aug 20 around noon. Been eagerly awaiting the release of the La Liga schedule and it came out recently that they play their first match on Sunday Aug 21!

Ok now I want to get tickets...they aren't yet for sale on the official site although I have been reading that because it's not Real Madrid that it probably won't sell out and tickets will be available there. A little concerned because I don't know if the first game would make it more likely for demand to go up(they're playing Real Betis so I don't think the demand for the opponent will be there). Should I just wait or should I look at alternate sites like Viagogo? Right now cheap seats are like 80USD but I'd like something a little better and better seats are really pricey.

Also, I know the dates are subject to change...what are the odds it would be moved to a Saturday afternoon that would make it difficult to land, put my bag somewhere, and make it? I'm thinking because it's the first match that it should be pretty concrete but I have no experience.

And finally, any recommendations for a good place to sit that aren't absurdly expensive? I want to be in an area that has a good fan atmosphere...getting that experience is about as important as seeing Messi and Neymar live.

Thanks