Mishaal Rahman of XDA Developers has seemingly discovered Android 12's release date. Last year, Google rolled out Android 11 to Pixel devices on September 8, the same date that it released the OS to AOSP.
Based on the screenshot below, Google will push Android 12 to AOSP on October 4. Rahman notes that this is a tentative release date, and it is much later than Android 10 or Android 11's debut. If Google releases Android 12 publicly and to AOSP simultaneously, it should arrive on the Pixel 3 through Pixel 5a first.
Following that, Android OEMs will start distributing Android 12 to their devices, such as Samsung and OnePlus. The Pixel 6 series will probably be the first devices to launch with Android 12, as has been the case with previous Pixel smartphones.
An October 4 release date would be before the date from when the Pixel 6 series is rumoured to be available to pre-order. According to Jon Prosser, Google will open pre-orders on October 19, with a full Pixel 6 series unveiling presumed to be occurring before then.