From Rolling Stone Magazine.com:

The Harlem Cultural Festival attracted everyone from Stevie Wonder and Nina Simone to Jesse Jackson and Marcus Garvey Jr., but quickly faded into obscurity. Fifty years later, a rediscovery is finally underway.

In October 1969, the writer Raymond Robinson took to the pages of the New York Amsterdam News, the city’s leading black newspaper, to pose a question. That previous summer, Harlem’s Mount Morris Park had hosted a series of free Sunday afternoon concerts, known collectively as the Harlem Cultural Festival, which featured a startling roster of artists, including Nina Simone, Stevie Wonder, Sly and the Family Stone, B.B. King, the Staple Singers, the 5th Dimension, and Gladys Knight and the Pips.“The 1969 Harlem Cultural Festival was, indeed, a meaningful entity,” Robinson wrote, “but was it fully appreciated?”

The series had been an unprecedented success, with combined attendance numbers [[roughly 300,000) that nearly rivaled those of that summer’s other unexpected musical phenomenon, Woodstock, which took place 100 miles north. As was the case with Woodstock, a filmmaker — Hal Tulchin — had captured the entirety of that year’s Harlem Cultural Festival, confident that the combination of the music [[Nina and Stevie) and the setting [[a post-’68 Harlem reeling from the assassination of MLK) would add up to a feature-length film that could cement the series of uptown Manhattan concerts as generation-defining events.

Read more here:
https://www.rollingstone.com/music/music-features/black-woodstock-harlem-cultural-festival-history-859626/