This is a friendly reminder of the critical importance of checking the opening times of all attractions important to you if you are planning to travel soon. If you're a pack-everything-into-the-minimum-number-of-days traveler, you should probably make a preliminary check before deciding how many nights you'll need in sight-heavy cities.
I know many if not all the Smithsonian Museums here in Washington DC are open more limited hours than usual--both fewer days of the week and fewer hours of the day.
I haven't yet actively sought out that sort of information for my own trip (still not sure where I'm going), but I just took a look at the website of the Pinacoteca di Brera in Milan, arguably the most important museum in the city. My old notes show an operating schedule of 8:30 AM to 7:30 PM Tuesday - Sunday from some time in the past. The current hours are:
Tuesday 9:30 AM - 2:30 PM (closed in the afternoon)
Wednesday and Thursday 12:30 - 5:30 PM (closed in the morning)
Friday - Sunday 9:30 AM - 5:30 PM (not bad, but 3 hours shorter than 8:30 - 7:30)
Per the common procedure in European museums, they stop allowing entry a full hour before closing time.
I can imagine someone arriving in Milan on Wednesday afternoon with a ticket in hand to see The Last Supper and planning to go to the Brera the next morning before moving on to Florence, Venice, etc. on a 1 PM train. Oopsie.