Yes it could be a bad time to invest in stocks. It all depends on what kind of stocks you are investing and how much money you invest with. Though I do think that it is not a bad just because it is the month of September. It could be bad or good in any month.
I don't think every September is a bad time to invest, but I certainly think that this September would be a bad sign. All stocks are very expensive even the treasuries and all signs point to a correction in the new future. There isn't a bargain to be found and it is never wise to buy high.
Yes, September is a bad time to invest in stocks, because historically businesses use September to make a lot of hard decisions. It is not a reliable enough time to bet on a stock, unless you have specific information that a stock is about to do well. You are better off waiting until things mellow out in October.
No, Any time you want to invest is good. I think anytime is a good time to invest in stocks in toady's market. I don't think a certain month or date makes a whole lot of difference in the long run. I feel that the best thing to do is invest in something you believe in.