According to XDA's Mishaal Rahman, Google is going to release the AOSP of Android 12 on October 4th. We can also expect stable Android 12 release on the same day.
IMG: Unsplash
Release Date
As usual, eligible Google Pixel phones will get the stable Android 12 update first. And later other phones will also get the update as per their official schedule.
Credit: @MadebyGoogle
If you are running a beta build on your Android phone, you can remove it using this guide to prepare for the stable build.