On the same day in November 1967 as The Moody Blues LP 'Days of Future Passed' was released, so was the single from the album 'Nights in White Satin'. Making the UK chart in January 1968 and reaching No.19 in February.  

Both the album and single having only moderate success upon release, 'Nights in White Satin' has enjoyed a recurring chart presence over the years. In 1972 following steady FM radio airplay and the success of this single, the album became a top ten hit in the US.
'Nights' was a hit again in the UK and Ireland in 1979 reaching numbers 14 and 8, respectively. The song charted once again in 2010, reaching number 51 in the UK Official Singles Charts. 
With hindsight 'Days of Future Passed' has since been listed among the most important albums of 1967 by Rolling Stone magazine, cited by the Rock and Roll Hall of Fame as an example of one of the first progressive rock albums and one of the first concept LP releases.
The "orchestral" sounds in the main body of 'Nights in White Satin' were actually produced by Mike Pinder's Mellotron keyboard device. One of the founding members of the band, sadly Mike's 'Days Passed' only last month, but as a pioneer of the Mellotron his playing would become The Moody Blues' signature sound.

With the instrument's instruction to 'play slowly' here's a rather flaky 'sound wise' 10 minute video that's very informative called "A Brief History of the Mellotron" Click here. 

The phrase “No Prog Rock without mellotrons” will clue you in to its main content.