The chat is on October 5 at 2:00 PM PDT. That's 5:00 PM on the East Coast, 9:00 PM GMT, and you'll have to figure it out for yourself for the rest of the world.
You can submit questions ahead of time in this thread, and the chat will be right here.
Actually, that's probably important enough to give it its own paragraph: the chat will be right here.
If you miss the chat, a complete transcript is right here!