Good question....wish I knew the answer....am planning on putting out some clover and winter rye, but am going to wait til end of AUG first of Sept. and see what happens if anything...
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It normally becomes a surer thing you haven't wasted your time and money if you wait until mid-September because of typically greater soil moisture by then. However, and there's no way to predict in advance, you may get greater growth (more forage mass) with a late August planting IF, IF, IF you time it right and receive adequate rainfall thereafter. Time it wrong and not get enough rain, you've wasted both your time and money. Less risk in waiting until mid-Sept.
"Timing it right" usually means planting immediately before a good rain, then getting another good rain soon after germination.
If you have both bottomland and ridgetop plots, I'd tend to plant the bottomland plots earlier, save the ridgetop plantings until mid-September to mid-October. Seems I'm often still at it even in late October, all too often re-planting the early plots that failed to get enough rain after the first planting.