We have a trip next month and this was our experience. That date effective means that's the date the you lose the ability to customize them and you'll get gray ones when you arrive at the resort. Once you've customized all your bands, Disney views them as ready to ship. Our "Customize By" date was August 1st for a check-in on the 12th.
I had heard that they normally ship them around 30 days before your check-in, but that date came and went for us with no shipment notice. Then I noticed a small "hiccup" in the process. One of our sons wants a gray band. Since that's the default color, I never opened his Magic Band record in order to then simply "save" the default details. Apparently this means that his band had not been "customized" and therefore our order was still "pending". Once I opened the record and then just hit the Save button without making any changes, our order was then completed and shipped the next day.