For most opera houses in Europe, including the Paris Opera, the performance season ends in June or July. New seasons normally begin in September. This means that in August, the opera houses are dark and the artists go on holiday or tour.
Unfortunately, that's also the time when many Americans take their vacations. If you're traveling to Europe in August, you likely will not be able to attend an opera or ballet at this time as part of the traditional opera house season. However, other options are available.
During August, opera companies offer summer festivals, often in outdoor settings or in historic locales, featuring guest artists. Also, many opera houses conduct guided tours as well as have museums displaying artifacts from decades, even hundreds of years of performances. The museums are open to the public in the summer months, including August.