I just returned from hunting buffalo during the first 10 days of September not far from where you are going (the Binga region of the Zambezi plateau). JLS is right on the money: cool mornings and warm days, though never too hot. There are few leaves on the trees giving you good visibility and fields of fire; of course, the downside is that what you can see, can also see you. Plus, the dead leaves can betray your stalk.

However, all things considered, it is the perfect time of year with few mosquitos or other bugs (apart from mopane bees which are ubiquitous). My PH and his assistant both said that their personal preference for safari is early-September. You'll be wearing shorts and short sleeves during the day and evening, but be prepared with fleece pants and jacket for the early morning drive when you'll likely be sitting outside and above the 4x4's cab. You may even want to bring a watch cap and gloves for those early mornings -- I wish I had!

The other issue that you may hear concern about is that hunting so late in the season may mean that the best trophies will have already been taken. If my experience is any guide, I took the largest buffalo of the season in our concession on the 3rd of September! So from my viewpoint, worry not and have a great trip!