There are two reasons to applaud at an HPO concert: as a greeting, and to show appreciation. You applaud to greet the Concertmaster when they come onstage (our Concertmaster Stephen Sitarski is the violinist who sits in the first chair of the first row of the Orchestra). You applaud again a few moments later when the Conductor comes onstage. Any soloists who will be performing will usually come onstage with the Conductor; you are encouraged to applaud to greet the soloist or soloists as well. You do not applaud again until the end of each piece of music, to show your appreciation to the performers.
Some longer pieces may have several sections, or movements, separated by a brief, silent pause. The audience does not applaud between movements of a piece. The program will list the movements in each piece, so you will know how many there are; applause is usually reserved for the end of the last movement. If you’re not sure that it’s over just follow the rest of the audience or wait until the conductor turns around to bow.