I can say this: we go on the weekend closest to my mom's birthday (Sep. 11), and every year the weather has been absolutely perfect. We couldn't ask for anything better. Couple that with the crowds being the best they are all year, and you have a perfect trip, in my opinion.
It's easy enough to be pleasant
when life hums along like a song.
But the man worth while
is the man who can smile
when everything goes dead wrong.
The 1st half of September is usually pretty hot. The second half of September and the first half of October is unpredictable.
The one thing you need to consider at DL once the fall approaches are the return of the Santa Ana Winds. This more than anything causes the fireworks to be cancelled, just to let you know. So if you are staying more than one day and plan to see the fireworks it's always best to try and see the the first night you are there or risk not being able to see them at all.