Canceled reservations are "released" immediately (or close to it, usually).
The 6am thing is with reference to the "next day". The system "rolls over" to the next day at 6am Eastern. For example, if you're 180 day mark was tomorrow, from midnight to 5:59 am (Eastern) it would still be today and you couldn't book yet. Once 6am hits, the system does its roll over thing and then you (theoretically, barring any system issues) would be able to book.