I'm planning a trip to Belgium and The Netherlands next September. Most of the popular museums are closed on Mondays, but not so in Amsterdam. Will I experience larger crowds at the Rijksmuseum, Van Gogh Museum, or Anne Frank House on Mondays?
The major museums in Amsterdam are open seven days a week, so Mondays shouldn't be any busier than any other weekday.
Be aware that Amsterdam hosts the International Broadcasting Convention Sept. 8-13. It's one of the largest conventions in the world and hotel rates will be sky-high during this period.
We were in Amsterdam last December. My advice... go to the Rijks and/or Van Gogh first thing in the morning. Get there just before they open. Otherwise, the line to purchase entry will be intimidating and the crowds will be annoying. (Alternately, you could try whichever night they have late hours... that worked for us at the Louvre.) For Ann Frank, go online and reserve/purchase your ticket ahead of time. Of all the lines we saw, Ann Frank house was the longest. We paid a little extra to have an English pre-tour description/discussion. It was very interesting and allowed us to pass the lines and enter the house with ease.
You could book tickets to the museums online first, it helps to beat the queues.