From July 2 to Aug. 21, the festival orchestra and chorus will return to their home at Pritzker Pavilion, marking the unofficial opening of public summer events at the venue. Though safety protocols will transform the festival’s sound and duration, Grant Park’s core mission remains intact: The 2021 season includes a healthy offering of programmatic rarities, the chorus will sing its usual six programs, and, yes, lawn spots will remain free of charge.