It completely depends on you and your situation. If you would like that space filled for income purposes then absolutely do not wait and get your ad up. It is not your responsibility to save a "maybe" space for them, unless they are willing to pay for it. In fact, I would put up your ad regardless because you never know, an amazing family may come along and be a perfect fit! And yes, an October ad would work nicely for you to nicely transition your baby in before transitioning another!