Yes, you can parboil potatoes and keep them until the evening... as the others have said, just keep them covered, cold and dry (ie not sitting in water but definitely in either a cold place or in the fridge) until you can roast them in the evening. You might like to rough up the edges of the dry potatoes before you drop them into hot oil... then they get a lovely crispy skin.
Parnsips - I wouldn't bother parboiling them but I would peel them, slice in half lengthways, remove the bitter heart and then place the raw slices in a bowl of water and pop this into the fridge until you are ready to dry them out and put into hot oil.