When 'Parsifal' was first performed at Bayreuth in 1882 there was some
confusion about when to applaud.  At the end of the second act there was
much applause and shouting, at which Wagner got up in his box and called
out to the audience that he had asked for no curtain calls until the end
of the performance.  At the end there was silence until Wagner got up and
said that he had not meant that they could not applaud, after which there
was enthusiastic applause and confused curtain calls.  By the second 
performance various accounts of what he had said were circulating.  Many 
thought that Wagner had asked for no applause until the end of each 
performance and therefore the first two acts were received in silence 
(except for Wagner himself shouting "bravo" at the departing Magic Maidens, 
for which he was hissed).  At the third and subsequent performances there
was no applause at the end of the first act but applause after the second
and third acts.  This became a Bayreuth tradition that continues to this
day.  Wagner gave the custom his approval, saying that applause was not
appropriate after the quiet ending of the first act, but the claim that it
was his idea is untrue.

The tradition of not applauding at the end of the first act of 'Parsifal'
is a Bayreuth Festival tradition. Therefore it does not apply in ordinary
opera houses. Sometimes, in some houses, there is a note in the programme
asking for no applause at the end of the first act; but in the absence of
any such request it is entirely up to each member of the audience whether
to applaud at the end of the first or subsequent acts. Please do not hiss
or "shush" those who choose to applaud.   Above all, please do not follow
Wagner's example and shout "bravo" at the end of the scene with the Flower
Maidens!