The Official Air Date is yet to be announced but a month has been given. A Tweet was made earlier saying that Doctor Who will be back this September!
Series 9 is looking like it will be airing later than Series 8. That is if this is official. Series 8 of Doctor Who aired on August 23rd 2014 with Peter Capaldi's debut episode "Deep Breath".
Series 8 was first aired on August Bank Holiday weekend. If this September date is incorrect, will Series 9 follow in the same way? Or maybe it will be September after all.
Doctor Who Producer Nikki Wilson posted this on Twitter- “We’ll be back in September”. This was in a response to a fan question. But, the Tweet was later removed. So this information might actually be false. Or, it isn't supposed to be released yet.
We'll have to wait and see. But in the meantime, the Doctor and Clara are back in September in "The Magician's Apprentice".