As it has in the past, Microsoft will be conducting maintenance on Xbox Live in the early morning hours (US time) tomorrow. This will affect Xbox 360 games, including those being played on Xbox One through backwards compatibility.
Starting at 1 AM PT / 4 AM ET / 9 AM BST on June 20, Microsoft will perform "scheduled maintenance that will affect Xbox 360 titles and services." This won't impact Xbox One games, but any 360 games being played on the system through backwards compatibility may run into issues. Specifically, Microsoft says that "you may be disconnected from Xbox Live one or more times."
It's not the first instance of this type of maintenance, and it should be a fairly minor issue depending where in the world you are. It's slated to last for only two hours, ending at around 3 AM PT / 6 AM ET / 11 AM BST, barring any unexpected issues.
While recent additions to the Xbox One backwards compatibility catalog have been slow, E3 did bring exciting news. Microsoft confirmed that support is coming for original Xbox games, including Crimson Skies: High Road to Revenge. Physical Xbox discs will work, and it will even be possible to system link for multiplayer across Xbox, Xbox 360, and Xbox One. A specific launch date for Xbox backwards compatibility has not yet been announced.