As I recall, Season 6 prepatching started nearly 2 weeks before launch. I suspect this is to give everyone ample time to get the patch without totally zerging the patch servers. Especially since patching is incremental, every time they push another build out to Tribble, the prepatch can be updated too, keeping everyone up to date and able to start Season 7 on launch day with as little delay as possible (aside from the inevitable login queues as everyone and their vulcan half-brothers login at once).
This is why picked the week of the 22nd as the live date for S7 when people asked me.
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