From Piazza Grande, Modena's main square, you can enter the Duomo through this beautiful doorway, flanked by two lions. There's a closeup of the lions in the next picture.
The cathedral is open daily, 7:00-12:30 in the morning and 3:30-7:30 in the afternoon (2006 times) except during mass. Admission is free but there's a charge to go into the Cathedral Museum (closed Mondays) and the Ghirlandina Tower (closed November - March and August).