There is no set time for postings for winter 2021 to start appearing. Each school has their own timeline and process for when postings go up. It is typically earlier for graduate programs and later for undergraduate programs. For example, Carnegie Mellon University's undergraduate application for winter 2021 is not open until September 1, 2020. Whereas, their graduate application for winter 2021 opened on May 1, 2020.
It is best to check with the school's admissions office or website for specific timelines. You can also check out this forum thread which discusses when people started seeing postings for winter 2021: