The golden age of progressive rock began on this day in 1969 with the release of two classic albums, Frank Zappa’s Hot Rats and King Crimson’s In the Court of the Crimson King.1
Also released on that day was the Kinks’ Arthur. It doesn’t quite qualify as “prog rock,” though it was a “concept album.”
I suppose I ought to comment about the historical significance of these works, with analysis of the musical techniques employed and explication of lyrics2, plus some personal notes…. Nah. The music speaks for itself.