Vienna is a very happening city, alive all year round. Festivities and artistic performances are a common and popular sight anytime of the year.
The weather of Vienna is a bit extreme, with winter temperatures dropping down to below zero degrees Celsius, and July being a milder month, with temperatures ranging in the 20°s C.
June and September should be the best times to visit the city, in terms of weather and not surprisingly, there are a lot of visitors during these months. So if you want to avoid the horde of tourists, and to enjoy serenity in solitude, it's better to opt for the spring, during April-May, and autumn, during October-November, when tourists are at a minimum.