As said in the usage notes "This question is often asked with the sole purpose of confusing who hears it." This phrase is neither a greeting nor has another understandable meaning other than confusing people; although puns like this are rather common in Portuguese. Usually, there are common, considerably clever and catchy answers to them, but I don't know any such answer to setembro chove? even after researching.
A person may, for example, simply answer in serious tone "It may rain or not to rain, depending on the place." or in annoyed tone "Don't direct this gibberish to me, seriously!" but these answers were invented by me now.
To be precise, you may respond almost whatever way you want. The only replies expected to avoid are "What?", "What the hell is bruxove?", or any related expression of lack of attention or desire to rationalize out of the blue, if you are interested in not being "owned" by this pun. --Daniel. 23:47, 15 March 2010 (UTC)