Naples Museum

Yes, it's true. From the website:

"Visiting hours and prices
HOURS
The Museum is open from Monday to Sunday from 9.00 a.m. to 7.30 p.m.
It is closed on Tuesday.
When Tuesday coincides with a holiday, it is closed on Wednesday.
It is closed on December 25th and January 1st.
The ticket office of the Museum closes half an hour before.
All Thursday from May 12th to September 29th 2016 the National Archeological Museum of Naples is open until 11.00 p.m. The last admission is at 10.15 p.m."

Yes, it's closed on Tuesday.

https://www.museoarcheologiconapoli.it/en/tickets-and-info/museum-hours-and-prices/

"I now need to rearrange my trip... "

At least you learned in time so you could rearrange your trip. When I went to Nancy, France, I really wanted to see the Ecole de Nancy museum. It turns out it's closed both Monday and Tuesday - which happened to be the two days I was in Nancy!

After that experience, I try to look at closing days for any place I really want to see. This saved me on my trip to the Basque country; I almost ended up in museum cities on a Monday, when almost all the museums in Bilbao, and ALL the museums in Vitoria-Gasteiz, are closed. Luckily, like you, I learned about this in time to rearrange my trip.

Also, the museum is not air conditioned. We were there in July and found it quite unpleasant. We endured and still enjoyed it but had we known, we would have come earlier in the day.

Beth

Which museum? there are many in Naples. Bound to find an alternate one to visit