Hi Lana!
For the permit, call the San Jacinto Ranger District at 909-382-2921 and they can help you sort out permit options. Generally speaking, it’s harder to get permits for holidays and weekends.
There is typically no water in September at the spigot in Round Valley, so plan accordingly.
The sun sets in nearby Idyllwild at 7:00 PM on September 10th, so if you get to camp by then, you should have enough light to setup your tent. Cooking dinner in dim light is easier that pitching a tent, so setup the tent first.
Happy to consult! Shoot me an email at [email protected] and we can set something up.