Carriers will start to take iPhone 6S/6S Plus pre-orders on Sept 11th.
A leak from a French site carrier was reported by Mac4Ever with the information of the pre-order date for September 11th of the both upcoming new iPhones 6S/6S Plus. (check picture below)
France has historically had the same pre-order and release dates as the US.
Apple will hold the announcements event on September 9th and will start delivering the devices in September 18th.
Note: European date formats run: day/month/year