Bologna hosts four to six trade fairs a year, during which hotel room rates rise dramatically. Some hotels announce their prices in advance; others wait to see what the market will bear. At trade fair times (dates vary yearly; check with the tourist office), business clients from throughout Europe book the best rooms, and you'll be paying a lot of money to visit Bologna.
An important note on parking: Much of central Bologna is closed to cars without special permits from 7am to 8pm daily (including Sun and holidays). When booking a room, be prepared to present your car registration number, which the hotel will then provide to the police to ensure that you are not fined for driving in a restricted area. Also, ask about parking facilities as well as the most efficient route to take to reach your hotel, because many streets in central Bologna are permanently closed to traffic. A permit is required to park in the center; hotel guests can purchase one through their hotel, if offered.
