Xbox’s Major Nelson has just announced that Microsoft is planning an Xbox Live maintenance tomorrow that will affect Xbox 360 titles and services. The maintenance is scheduled to occur between 2:00PM UTC and 6:00PM UTC (9:00AM ET and 1:00PM ET), so if you're planning to play Xbox 360 games during that period, be aware that you may be disconnected from Xbox Live.
For Xbox One owners, the scheduled maintenance will also affect all backward compatible Xbox 360 games on the console, though Xbox One games and Xbox Live should work as usual. There are now over 500 Xbox 360 and original Xbox games that are backward compatible on Xbox One, and the ability to play older generation games is definitely one of the console's killer features.
The last time Microsoft did such a maintenance was in July 2018, and that time Xbox 360 titles & services were only impacted for an hour and a half. If you're encountering any issues tomorrow, make sure to check the Xbox Live service status on this page.