The event will be held on September 29th and the most of the attention will be on the duo Nexus smartphones, the Nexus 5X and Nexus 6P while the owners of the older Nexus devices are mostly just interested in when they will be getting the new version of Android, Android Marshmallow.
Telus, a Canadian mobile provider, has leaked the tentative date for the Android M for Nexus 5 and Nexus 6 which is October 5th. The information now has been taken down but check the screenshot of it:
Considering that is the timing of a carrier based update, for unlocked devices, it should be even faster. It can be available anytime as soon as Google says the word on September 29th.
So be ready and prepare your phone.