If they are forming now they will definitely be ready by September. I wait until the tendril next to the gourd turns brown before I pick mine. But it doesn't always have to be completely brown before I pick them. Also when they are mature they lose the high shine that the immature gourds have. My Advice is to pick them as they are ripe and not leave them on the vine. This will allow the vine to produce more gourds and some gourds when left on too long will change to an undesirable color. Like solid green gourds will turn orangish and white ones will turn yellowish or a hint of green.