Spring is the obvious time to visit, especially if you want the Kentucky Derby experience — the race is the first Saturday in May, and the two weeks of Derby Festival leading up to it transform the city. Hotels book out months in advance during Derby week; prices are high. April is pleasant without the crowds.
Summer is hot and humid, but the city keeps busy with outdoor concerts and festivals. Fall is excellent — mild weather, good foliage in the parks and cemeteries, and bourbon tourism at its most comfortable. Winter is cold but rarely severe, and the lack of crowds makes for easy access to museums and restaurants.