All summer Ravinia has been celebrating the 25th anniversary of its summer conservatory, Ravinia’s Steans Music Institute. Perhaps the biggest celebration yet comes tonight, Aug. 12, with a concert that boasts nine world-premiere songs, commissioned for the milestone year. Tickets are just $10, and all ticket-holders are invited to a post-concert party. The songs are Jake Heggie’s By the Spring, at Sunset, Aaron Jay Kernis’s setting of Walt Whitman’s Clear Midnight, Ramsey Lewis’s Quiet Moments, David Ludwig’s Still Life, Stephen Paulus’s Was It All a Dream?, Augusta Read Thomas’s Twilight Butterfly, and a three-song cycle by Roberto Sierra called Décimas.