Was reading a bit yesterday on Wikipedia about the premier performance
of Richard Wagner's opera Parsifal at the Bayreuth Festspielhaus in 1882.
Evidently some confusion arose at the performance as to when to applaud
and when not to applaud:

"At Bayreuth performances audiences do not applaud at the end of the
first act. This tradition is the result of a misunderstanding arising
from Wagner's desire at the premiere to maintain the serious mood of the
opera. After much applause following the first and second acts, Wagner
spoke to the audience and said that the cast would take no curtain calls
until the end of the performance. This confused the audience, who
remained silent at the end of the opera until Wagner addressed them
again, saying that he did not mean that they could not applaud. After
the performance Wagner complained "Now I don't know. Did the audience
like it or not?" At following performances some believed that Wagner
had wanted no applause until the very end, and there was silence after
the first two acts. Eventually it became a Bayreuth tradition that no
applause would be heard after the first act, however this was certainly
not Wagner's idea. In fact during [one of] the first Bayreuth performances
Wagner himself cried "Bravo!" as the Flower-maidens made their exit in the
Second Act, only to be hissed by other members of the audience. At some
theatres other than Bayreuth, applause and curtain-calls is normal
practice after every act; other major theatres, including the Metropolitan
Opera in New York, follow the Bayreuth custom."

It would be my view that there should be no applause whatsoever during
Parsifal, even at the end. And if this did not make the performers happy,
then it would be the job of the music director to placate their egos.
There is a solemness to Parsifal, a feeling of sacredness. And I can no
more imagine an audience applauding at the end of it than I can the
members of a church congregation applauding the priest or minister at
the end of a service. To not applaud might not have been Wagner's
intention. But I think it is fitting.

Many opera houses today have a large digital screen above the stage that
is used to give translations of the libretto as an opera is sung. I suppose
it would be no trouble to program it to read "Applause" when the time
came for that type of thing. Or for that matter, during a comic opera the
screen could, where appropriate, read "Laugh."

On second thought, forget all that. I don't know what I could have been
thinking. Too much Gatorade on top of my Gurnemanz, perhaps.