Assuming you mean "hot" as in temperature.
For Europeans, I suggest Egypt. Especially if you can wrangle a day trip or camel train journey into the Sahara. Most of Egypt is well set-up for western visitors, in terms of tourist attractions and facilities.
Go to inland Egypt if you want really hot. Luxor, Valley of the Kings, anywhere in the south of the country.
Similarly, one could try to visit the wilds of the inner reaches of the Saudi Arabian peninsula. But you really do have to be Muslim to go there. The ordinary people are hospitable to all visitors, but the zealots and nutters have this idea that their country is not for sharing with the infidels.
For North Americans wanting a hot vacation in September, try visiting central America.
Other places that are potentially very hot in September include Houston, most of Africa, most of India, Iraq, Bangladesh, Thailand, Indonesia, Malaysia and northern Australia.
Wherever you go, check that you have all the right vaccinations. "Hot" places tend to be just right to breed lots of insects, especially biting and disease-carrying ones.