It says Wednesday in Kanji. Sunday is written in red. More correctly, it is the 3rd day of the week, thus the 3 slashes
I'm pretty sure the Rolex Cellini Prince (not an Oyster case) had a displayback in the early 2000s. The Platinum Daytona seems to be the first Oyster case with one, and the new Perpetual 1908 (Cellini line replacement?) has one. Since most Rolexes are tool watches, and since they haven't historically decorated their movements, I guess they've found no need to put a displayback on prior. I'm not sure I'm a fan, actually. I love displaybacks on watches where special attention is paid to the movement - especially those with microrotors so the movement is less hidden. Check out Czapek's for what I mean. For Rolex though, I kind of expect the stout metal back.