Is it open all year round or only during certain periods like the floors?
I believe the library is open year round at the same times as the Duomo itself.
I am not sure. I was there in April, and aside from the altar renovations, everything was "open" & accessible to visitors
i believe its opened all year! but i can't assure you!
It is should be open whenever the cathedral is - though as with almost anything in Italy, they may be doing restoration, so it could of course be subject to closure.
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