Before there was glam rock, there was “progressive rock,” that nebulous genre that saw the music of ’60s psychedelia taken to new, expansive dimensions of lyrical poetry and symphonic instrumentation. The groundwork was perhaps laid by the sweeping sonorities of Pet Sounds and Sgt. Pepper’s Lonely Hearts Club Band, but if a single album were to have firmly established what “prog rock” was, that album was The Moody Blues’ Days of Future Passed.