The Stones recorded this in the summer of love and released it as a single in December 1967. Incredibly, it did not chart.
The song was written by Jagger and Richard, the strings were arranged by John Paul Jones and it features a mellotron played by Brian Jones.
The piano was played by Nicky Hopkins, who began his career with Screaming Lord Sutch (whom I remember applauding around the hall when he came to the count at Littleborough and Saddleworth in 1995). He later became one of the great session musicians of his era.
When Hopkins died in 1994 Ray Davies of the Kinks wrote of him:
The Kinks had always used a piano to help build the wall of sound associated with our early hits "You Really Got Me" and "All Day and All of the Night." Now we were making our third album and our producer, Shel Talmy, thought we should hire someone who could contribute more than just background chords.
Nicky, unlike lesser musicians, didn't try to show off; he would only play when necessary. But he had the ability to turn an ordinary track into a gem - slotting in the right chord at the right time or dropping a set of triplets around the back beat, just enough to make you want to dance. On a ballad, he could sense which notes to wrap around the song without being obtrusive. He managed to give "Days," for instance, a mysterious religious quality without being sentimental or pious.