Depends on the style of deadlift you are talking about. If you mean a stiff legged or Romanian stlye deadlift, then it is on leg day. If you are doing standard, you can do them w/ legs or back. If you want to do them w/ back, you can just do partials from the knee.
actually if you use the search, this topic has been discussed in detail (with pictures attached ) within the past month or two... i'm sure you'll find all the answers you need there...
i actually do deads seperate from legs,i do them with back.i also do quads and calves together only. a bodybuilder friend of mine had me seperate hams and quads and i get better workout that way.......