Summer is the peak tourist season in Nashville and Memphis. Unless you're specifically visiting Nashville for the Country Music Fan Fair in June, or going to Graceland for Elvis Week in August, you might want to avoid traveling during these times, when hotels sell out and prices go through the roof.
Summer is also when both cities experience their worst weather. During July and August, and often in September, temperatures can hover around 100°F, with humidity at close to 100%. (Can you say "muggy"?) Spring and fall, however, last for several months and both are quite pleasant. Days are often warm and nights cool, though during these two seasons the weather changes, so bring a variety of clothes. Heavy rains can hit any time of year, and if you spend more than 3 or 4 days in town, you can almost bet on seeing some rain. Winters can be cold, with daytime temperatures staying below freezing, and snow is not unknown.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.