Is it OK to just turn up or is it best to buy tickets to the jazz festival?

I went to all the concerts of the 2009 festival.
Most nights were sold out except Monday and Thursday so yes,
it is necesary to buy tickets.

I bought tickets from TICK TACK TICKET:
https://ticketmaster.es/

I am in Granada so I picked the tickets up from  
the box office at Teatro Isabel La Católica in the centre of Granada.