Close friends having their wedding same month as ours, what is the etiquette?

You have a reason why it needs to be in August, you should be able to have it then. If she just wants it then, then she needs to know that you have no choice but August, and if she does it in September, that it would be better for everyone and it'd only be a couple weeks of waiting. If she flat out refuses, then I guess thats what happens. Of course you don't need to ask permission, thats rediculous. You should tell her you need August, and be reasonable about it.