Hi Welshlass,
Unfortunately I think you will have missed the festivities, Salou's main summer festival is called 'Nits Daurades' or 'Golden Nights'. The festivals finale is tomorrow night at 10.45pm Spanish time when they have the best firework display I have ever seen. The festival started on 08 August 2009.
Maybe someone can advise if there are any festivals in the surrounding towns such as Tarragona, Reus or Cambrils?
If you want to see what the Golden Nights festival was about, then here is the link to the website but you'll have to know your Spanish

I'm not sure why there is no English translation? there always has been in previous years.
http://www.salou.org/2009/NITS_DAURADES_09.pdf
If I manage to find out if there are any festivals in the nearby vicinity during your stay I will post on here again.
Mark