Saturday brings the autumnal equinox, which means it's time for music, poetry and communal singing at the 9th annual Revels RiverSing at the Charles River. Revels music director George Emlen hosts the free outdoor celebration featuring over 100 chorus members, the Revels Children’s Chorus, musician David Coffin, the Second Line Pleasure Aid and Social Society Brass Band, sax man Stan Strickland on the river (right) and more. Did we mention it's free? There will be groups sings of about 20 folk songs (lyrics available onsite and at www.revels.org). Actors’ Shakespeare Project’s Steve Barkhimer and Jennie Israel and Cambridgepoet Toni Bee will recite. The main event is at 6 p.m., but festivities begin at 5 with family fun in Harvard Square's Winthrop Park, followed by a procession to the river. This is fun for anyone who likes music and ritual, and those with an anthropological interest in Cambridge's unique culture.