Final answer:
The event in Music's history that ended on August 17, 1969, after three days was Woodstock, which symbolized the counterculture movement and cultural freedom of the 1960s.
Step-by-step explanation:
On August 17, 1969, the "grooviest" event in Music's history that came to a close after three days is famously known as Woodstock. This iconic music festival drew nearly 400,000 people to a farm in rural Bethel, New York, who experienced a dynamic mix of music, art, and community.
Aside from the vast number of attendees, Woodstock was notable for the free-wheeling spirit, widespread use of marijuana and psychedelics like LSD, and for symbolizing the counterculture movement of the 1960s. It featured thirty-two acts and embodied the cultural independence and freedom of American youth during that era.