In the past, when you wanted to book a camp site in Yellowstone National Park, we have been told that you had to wait till January 1st in order to make reservations for that year.
This is no longer the case. Now, you can reserve even further in advanced. They now take reservations starting on May 1st for the following year. So if you want to book a reservation for Summer of 2021, you can start booking on May 1st 2019.
Also, the latest word on Fishing Bridge is it should be open by September 2020, check the Yellowstone National Park website for more information.