Final answer:
Woodstock is the epic rock concert of the 1960s that had over 400,000 attendees and is often considered a defining moment of the counterculture era.
Step-by-step explanation:
The epic rock concert of the 1960s that had over 400,000 attendees and is often considered a defining moment of the counterculture era was Woodstock. Held in mid-August 1969 in rural Bethel, New York, Woodstock attracted a massive crowd that exceeded the expected fifty thousand. Thirty-two acts performed over three days, and the event became a cultural touchstone for the generation coming of age in the 1960s.