Bay Seniors (in fact every senior I’ve ever met no matter how solid their bank account) love getting things for free. This Sunday will be the last no-cost event of the 2011 Stern Grove Festival out in the dark,cool, not to say frigid, groves on 19th Avenue and Sloat Boulevard.This time the San Francisco Opera Orchestra is on, with internationally applauded mezzo-soprano Dolora Zajick up front. The aria program has not yet been announced. It starts at 2 but if you want to snag a decent seat come at least an hour early. You may not find one even then so beach chairs, blankets, mufflers and mittens are all advised. Brrrrravo!