Concierge.com's insider take:
Every day is market day in Bologna. Locals and tourists alike haggle for fresh fruits and veggies, spiffy leather handbags, and antique furniture at these center-city street fairs. (Watch your wallet though: Pickpockets have been known to "shop" there too.)
The largest is La Piazzola (Piazza VIII Agosto, 348/006-2204, every Friday and Saturday), a great place to pick up shoes, crafts, fabrics, and a huge selection of vintage clothes. For produce and seafood, don't miss the Mercato di Mezzo (Via Pescherie Vecchie) and Mercato delle Erbe (Via Ugo Bassi, 051/230-186).
Collectors love bargaining for antiques and knickknacks at Celo' Celo' Mamanca (Piazza San Martino and Via Valdonica every Thursday) and San Stefano (Via San Stefano, second Sunday of the month except July and August). The Decomela market (Via San Giuseppe) is the place for arts and handicrafts.
There are ten regular markets and various seasonal markets selling holiday sweets and Christmas decorations. Check the tourism office's Web site for the entire schedule (iat.comune.bologna.it), click on the link for "events" and then go to "markets").