It doesn't really matter. Nothing is going to change over the weekend. I always think getting your name in earlier is better -- just in case someone else "beats you to it," but there is nothing magical about one day of the week over another.
However, trying to get someone's attention on a Monday might be difficult if they come in to work on Monday and find a lot of things that piled up over the weekend that have to be addressed before they can start on "new business."
I guess I would recommend Friday over Monday ... but I don't really think it matters that much if you are hunting for a job in the right way to begin with.