Well, that's what it is right now for the Dewey editorial team (save consulting assistant editor Giles Martin in Australia, who is probably already into Wednesday morning). But it's also the title of the sixth song in the set that the Moody Blues played last night in Pittsburgh. Listening to them, I couldn’t help but muse about how useful it is to have the note "Class here specific rock styles" under 781.66 Rock (Rock 'n' roll). In a show that ranged from pure rock ("I'm Just a Singer [in a Rock and Roll Band]") to spoken word ("Late Lament") to the something-for-everyone-in-one-song ("Question"), I thought about how hard it is to describe the Moody Blues beyond rock. The Manual note at 781.62 vs. 781.63-.66 Folk and popular music instructs one to use allmusic.com or a similar online source for help determining the style of music. Allmusic.com lists the Moody Blues in the Pop/Rock genre, with the styles of Prog-Rock, Contemporary Pop/Rock, Album Rock, Art Rock, British Invasion, Psychedelic, British Psychedelia, and AM Pop. I think this gives you an idea of why we decided not to subdivide 781.66 by specific rock styles!
The biography number for the Moody Blues is 782.421660922 (built with 782.42 Songs plus 1 General principles [from the add table under 782.1-782.4 Vocal forms] plus 66 Rock music [from 781.66, following the instructions in the aforementioned add table] plus T1—0922 Collected biography). For that last element, if you were tempted to add collected biography by geographic area and slip in T1—092242 (T1—0922 plus T2—42 England) or T1—092242496 (T1—0922 plus T2—42496 Birmingham), please don't do so. The Manual note at T1—0922 Collected biography instructs: "Do not use area subdivisions for collected biography of groups that have a strong collective personal identity, even though all the members are known to have come from one particular area, e.g., the Beatles 782.421660922 (not 782.42166092242 or 782.42166092242753)."
And yes, they did play "Nights in White Satin."