Bolzano is located in the Trentino-Alto Adige/Südtirol region of northeastern Italy. Ski country.
Summers in Bolzano are relatively mild, generally staying in the 80s range--but summer is the main tourist season in Bolzano, so it's pretty crowded. (You can escape the heat by taking a cable car into the mountains.) It rains more in the summer than any other time of the year in Bolzano, unusual for Italy. Winters are cold are generaly dry; January precipitation is sparse.
April through June is a good period to visit Bolzano. Wine lovers visiting the South Tyrolian Wine Route will prefer the fall during harvest season.
Unless you're someone who enjoys cold and often dark days, avoid January, which averages only about three hours of sunshine a day.
The climate charts below show the current weather conditions show the historic high and low temperatures and average monthly rainfall for Bolzano, Italy.