The entire pavement is uncovered for about two months each year, usually starting during the week after August 15. During the period when they are uncovered, the admission charge for visiting the duomo is raised. In 2012, the raised ticket price was 6 euro or 12 euro for a Cathedral Combo Pass.
If you plan to visit the other interesting monuments associated with the duomo, including the Baptistery and Crypt with its beautiful frescoes, it's best to buy a Cathedral Combo Pass, good for 3 days. Tickets or the Combo Pass can be bought at a ticket office near the cathedral or you can buy the pass before you go from Select Italy (details).
Hours: Monday - Saturday, 10:30 - 19:00 and Sundays and Holidays, 9:30 - 18:00
Location: Piazza del Duomo, a short walk from Piazza del Campo, Siena's main square
If you're in the area during the period the pavement is uncovered, don't miss the opportunity to see it.