Figure on it being available to Guests checking in from ABOUT the last Sunday in August to ABOUT the last Saturday in September, or maybe the one before that.
Then factor in that it's already been offered to Guests from the UK checking in even earlier in August, throughout the month of September.
Add in that it was already offered as the Bounceback/Future Stay promo to last year's September onsite Guests from the end of August throughout the entire month of September and even (I think) checking in as late as the first couple of days in October.