"How many days does the month of September have?"

Translation: Hoeveel dagen heeft de maand september?

[undergroundo]

Is it also grammatically correct to say "Hoeveel dagen heeft de maand van September?

[Kai_E.]

No, you don't use "van" in instances where the name of the month comes right after the word "maand".

• de maand mei
• de maand juni
• de maand april

etc.

[Splodgeit0]

I put the word van in where Cesar has and it marked it ad correct