Italy
It looks equally amazing from the outside, with its facade and obelisk, or the inside, especially the ornate ceiling. You're unlikely to have to queue to get in, in contrast to another cathedral in town. It's also the Pope's own church.
Near the San Giovanni Metro, or a ten minute walk from the Colosseum.
Open 9am-1hr before sunset
Enter from Via Sacra, Largo Romolo e Remo or Via del Foro Romano
This year it was from 2nd April to the 9th April and many of the major sights were free.
www.beniculturali.it/settimanacultura/stampa/GalleriaGrandiEventi/lazio.htm
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