The Duomo or Cathedral of Naples is on Via Duomo. From Via San Gregorio Armeno, turn right on Via dei Tribunali and then left on Via Duomo.
The Duomo is a 13th century Gothic cathedral dedicated to San Gennaro, Naple's patron saint. Inside, the Chapel of the Treasure of San Gennaro has Baroque frescoes and other artworks but most importantly it holds the saint's relics including two vials of his coagulated blood. A huge festival is held on September 19, the Feast Day of San Gennaro. Thousands gather in the cathedral and square outside to witness the the miracle of San Gennaro when the vial is taken out of its storage place and the blood (hopefully) miraculously liquefies. As with many Italian festivals there's much more than just the main event. Processions and celebrations go on for eight days. The miracle of the blood is also performed on the first Saturday in May.
The cathedral is steeped in history. On one side of it is the 4th century Basilica Santa Restituta, the oldest church in Naples, with stunning ceiling frescoes and columns believed to be from the Temple of Apollo. From the church, you can visit the archaeological area under the cathedral with remains from the Greeks to the middle ages. The 5th century baptistery has good Byzantine-style mosaics.
Currently the cathedral is open daily from 8:00 to 12:30 and 16:30 to 19:00 with slightly later closing hours on Sundays. The archeological area is open daily 9:00 to noon and 16:30 to 19:00 but closed on Sunday afternoons. See the Cathedral web site for updated information.
This ends our look at the historic center. The National Archeology Museum of Naples, well worth a visit, has one of the world's best collections of Greek and Roman artifacts and is close to the center. For more Naples sights and attractions see What to See in Naples.