Your life need not be wholly consumed by A-levels alone over the next couple of months - there still needs to be a balance between your studying and the rest of your activities, including fun things like concerts.
If the concert sits well with your work/life balance, then sure; go out and enjoy yourself. However, if you feel you should be cramming for all you're worth on the weekend before your A-level, I would promise myself a concert after the exams are over.
You know yourself best, so go with what suits you.