A joyous annual tradition, RiverSing returns to the banks of the Charles River in Cambridge on September 21 for its 11th year. Created by Revels, a performing arts organization dedicated to bringing people together through traditional music and dance, this free, family-friendly event celebrates the autumnal equinox with a colorful, puppet-filled parade, followed by the main event, a communal sing-along led by the 100-member Revels RiverSing Chorus of adults and children at the Weeks Footbridge. A slew of guest singers join the fun, as well as percussion ensemble Bloco AfroBrazil and acrobats and circus performers from Moonship Productions. What better way to say goodbye to summer and welcome the official start to fall? Join the festivities beginning at 5 p.m. in Harvard Square before the 5:45 p.m. parade, and get ready to sing your heart out at 6 p.m.