July 21st was just one of many highlights in Hendrix's 1967

1967, The Jimi Hendrix Experience played the first of three nights at the Cafe-a-Go-Go in New York City.

July 21-23, 1967 Jimi Hendrix Experience/Richie Havens/Jeremy and the Satyrs
This was the sort of legendary show that put Cafe Au Go Go on the map. The previous summer, Jimi Hendrix had been backing John Hammond--now he had returned as the hottest new act in rock. 
As if Hendrix's presence wasn't enough, Eric Clapton, recording with Cream in New York attends one of the shows, and jams with Hendrix (and on another night, he sits in with John Hammond at the Gaslight). The Mothers were still holding court upstairs at The Garrick, and Mitch Mitchell remembers sitting in with the Mothers one night. Mitchell thought that Hendrix probably jammed with the Mothers too. Eric Burdon and the Animals were originally booked, but seem to have been replaced by Hendrix. Hendrix and Burdon shared management (Mike Jeffferies and Chas Chandler), so it seems plausible.
Above entry taken from Rock Prosopography 101 blog

Other momentous dates you my remember in 1967 for Jimi was the release of a little album called Are You Experienced.  Or maybe the iconic photo of him setting his guitar on fire at the Monterrey Pop Festival?  Then just to round out a slow year he released Axis: Bold As Love which was another album that made use of new studio technology.  These two albums in 1967 had very uncommon sounds and blew people's minds.  Its hard to imagine that now,but how cool would it have been to be there and heard these albums for the first time in 1967.

Mike Smith, The Showbiz Kids