When is the best time to buy the new iPhone?

Hi. The next iPhone comes out on September 10th. I wanted to get one, but I was wondering when the best time to do that might be? On the opening day, I assume they are going to be selling a lot, but in the past, have they had them any cheeper on the first day, week, or something? Again, in the past, have they done any sells a week or two after it is released?

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