Update: Loran got it from The Midvale Cottage Post (specifically this post, but check out all the amazing 1920s stuff at that first link); I thought I had the original source can't believe I didn't click through. The pattern is from Ruth Wyeth Spears.
It's not a terribly useful dress, but it's oh so pretty. Chiffon. Floaty. I kept thinking this dress was familiar from somewhere...