Well, the Macs usually launch on Tuesdays, so that leaves 4 possible dates:
11/6
11/13
11/20
11/27
Thanksgiving falls on the 22nd this year and I would imagine Apple wants to get these out before Black Friday, aka the start of Christmas shopping time. Based on my theory, I would guess either the 13th or 20th. Again, this is just an estimated guess.
Tim Cook confirmed today that the November and December release dates for the new models will mean significant shortages for the robust demand the company is expecting headed into the holiday season.