Bratislava has four distinct seasons. The seasons here run just like one learns from a textbook: summers are hot, fall is beautiful with the foliage colors, winters are full of snow, and spring come bearing flowers.
The summer has average temperatures in the 80s and is the tourist high season. It is also full of festivals and events that tempt more people to come in. But another great time to visit is September and October. These months have great events as well. For example, the Bratislava Music Festivities series really gets rolling during these months. This musical celebration highlights the best of the best from Beethoven to classic and new jazz. Different philharmonic groups travel from all over even the U.S. to perform at these series. Another great aspect about this time of year is the celebration of the wine season. Many wineries open for tours and toast to the new wines.
Winters get a fair amount of snow and thus have cold temperatures but, the temperatures do average out above freezing. A thick jacket and warm scarf should be plenty to keep you toasty and a glass of hot wine will help as well. December is a great time to visit as you can experience the Christmas markets a tradition of the region. On New Year's there is a party on the main square.