Looking real quickly at the threads from last year, word of the 700 seem to start coming out the 2nd week of September
https://www.mobileread.com/forums/showthread.php?t=28964
So my guess would be that Labor Day weekend would be a good weekend to begin a "Coming Oct. 1 . . . " ad campaign.
Just guessing.

. . . . bigger screen
. . . . wifi / 3G