Spring (April to June) and autumn (September to October) are the best times to visit Bologna. The weather is pleasant and the city is lively without the summer heat.
Summers are hot and humid, with temperatures regularly above 30C. Many locals leave the city in August, and some restaurants close for holiday. Winters are cold and often foggy, with occasional snow from December to February.
Christmas time is wonderful -- visit the "Mercatini di Santa Lucia," one of the oldest Christmas markets in Italy, held in the streets around the basilica. In summer, restored silent and sound films are shown outdoors in Piazza Maggiore.