I honestly have no idea where September 15th comes from. Well here’s the source for September 30th:
A newspaper article from 1978, that’s pretty close to the actual date, so that means that it should be reliable.
Now let’s have a look at a calendar from 1978, the Saturday before October 3, was September 30.
That’s it. I believe this to be true. But everyone is entitled to their own explanation. And if someone is able to show me a source for the September 15 version, I might believe that. But so far that didn’t happen.