I think there may be an article on wdw that shows historially when parties were sold out. This year we are waiting until we have a solid weather reports a few days before hand for
MNSSHP. Our dates historically have never sold out anyways.. I am willing to risk that and buy a day or two before and know most likely we will not have torrential rains as we have done that twice and it ruined our party.. nothing worse than paying for that and being rained out.