Tickets go on sale depending on each competition.
In La Liga, tickets go on sale about four weeks before the start of the competition once the fixture list has been announced.
Do tickets for FCB v R. Madrid go on sale as part of the same process?
For FCB v R. Madrid, there are usually two sales processes: an initial one on the same day that all the other matches go on sale (only for non season ticket holding members), and a second one a few days before the match and always providing tickets have been made available via Seient Lluire (S.LL), which initially go on sale to members. If there is high availability, tickets are also put on sale to the general public.
In the Champions League (UCL), once the draw for the group stage has been made in early September, tickets go on sale. If Barça qualifies for any of the different knockout rounds, then tickets go on sale as soon as possible.
For knockout matches there are usually two sales processes: an initial one (only for non season ticket holding members) with a finite number of tickets as allocated to the club, and a second a few days before the match and always providing tickets have been made available via S.LL. which initially go on sale to members. If there is high availability, tickets also go on sale to the general public.
COPA DEL REY
In the Copa del Rey, tickets go on sale on the day(s) after the draw is made.
If the draw produces a match between FCB and R. Madrid then the sales process is the same as if it were a Liga fixture.
OTHER COMPETITIONS OR MATCHES
For matches in the Gamper, or a possible Spanish Supercup, tickets usually go on sale 3 or 4 weeks before the match.