Richie Havens may have kicked off musical proceedings with his opening solo set, but Sweetwater, truth be told, is the first actual musical group that played the Woodstock Music & Art Fair on August 15, 1969.  According to Woodstock Wiki, “Day Song” is the fourth song they played:

“Day Song”     Sweetwater     1969

Reprise would release “Day Song” as the B-side to “Without Me” in March of 1971.