When the Moody Blues entered a Decca recording studio in October 1967, they were a modestly successful British Invasion act with one likable Merseybeat hit single to their credit, 1965’s “Go Now.” Their contract with the label was about to expire and they owed several thousand pounds to Decca. In exchange for having the debt canceled, they agreed to make a rock version of Dvorak’s New World Symphony that the label wanted to use to showcase a new stereo audio format.

If record buyers were a bit slow to catch on, critics were even slower.

But the weeklong Decca session didn’t work out as planned. Instead of a Dvorak recording, it produced Days of Future Passed, an album that successfully fuses rock and classical in a performance that profits substantially from the contributions of composer/conductor Peter Knight and his London Festival Orchestra (a nonentity that was named for the…