The best time to visit Cincinnati is either late spring, of early fall. Yes, it can be hot and/or rainy that time of year, but generally those events are short, no prolonged heat and the coolness of a canadian front can still sweep through the Ohio Valley. However, as much as May is very nice, there is nothing like a Cincinnati Fall day. Typically, the last two weeks of September through the third week in October are ideal. Generally, warm sunny dry days followed by cool crisp nights, maybe a touch of frost, but in many years not even a frost has touched the ground. The beauty of fall leaves can be caught in fullout in early to mid-October adding to the general scenery. Temps in these two periods seem to be about 60-70 highs with lows ranging from the 40- to the low 30's.