In trying to organize my days in Florence in early September, it seems that the Duomo is closed on Sept. 8. I googled but couldn't figure it out and the official website http://www.operaduomo.firenze.it/eng...era/orari.asp? doesn't say why. Is this a religious holiday?

Just curious, thanks so much!

It is the feast of the Nativity of the Blessed Virgin Mary. Seems to be a big deal in some European countries.

Thanks! Do you think museums will be closed, or only churches?

It seems the Cathedral is open, but everything else is closed, i.e. the Baptistry, the Dome, the Tower and the Museum.
8th September is the Birthday of the Blessed Virgin Mary, patron of the cathedral, dedicated to St Mary of the Flower, i.e. the lily, the symbol of Florence. Perhaps the staff are given a day off!

From what I gather from the link you posted, the cathedral itself is not closed on September 8th, but only parts of it are.

It is, quite obviously, a big deal in a cathedral that is dedicated to the Virgin Mary. They will be having special services and such in there that day and won't want the usual hordes of tourists trampling around.

Of the many feasts of Our Lady, her Nativity is among the least commemorated in most of Europe. It's scarcely even heard of in the thousands of European churches dedicated to her: most use the Annunciation (Mar 25) or Assumption (Aug 15) as their Saint's Day (and most Protestant churches of St Mary regard Saint's Days as part of the superstition they got rid of 500 years ago anyway), so you'll find yourself rarely inconvenienced by anything closing for Sep 8

However, in Florence the (essentially civic) Festa della Rificolona is organised around that feast, involving processions past the cathedral. Which is why part of the cathedral's closed.

All of which said, lots of Europe has local festivals, ultimately derived from local harvest periods, in Sept. Rarely publicised outside the immediate area - but well worth seeking out wherever you might be.