Well technically I believe the song was Nights in White Satin, somewhere I'd love to spend nights with you. If we are sticking to Knights, then I'd have to say the Templars were the closest thing. They wore a white over-tunic emblazoned with a red cross while on the crusades. Though without a doubt it would have probably been Linen or if you think extravagant, maybe silk.
It IS NIGHTS in white satin... not KNIGHTS in white satin. But who knows, maybe some of those KNIGHTS back-in-the-day were wearing satin under their clothing.
Back when that song was written many grooms wore white or cream suits or tuxes. Maybe that had a something to do with the song.
I have been busy putting flooring in and painting for my friends in the last weeks.