Valve has announced that it will be delaying the Steam Summer Game Festival from its original dates of June 9th-14th to June 16th-22nd. Essentially it is a one week delay.
Nothing has changed from the structure of how the event will run. There will still be time-limited demos and live events with game developers. The new time frame and a brief explanation of the event can be found on the Steam Store page.
Valve did not issue a statement for why they were delaying the event. It is likely because of the protests in the United States and the world regarding racial justice and police brutality.
It appears that Valve's decision was sudden, and many did not know about the delay until they saw the updated Steam Store page for the event, or via tweets like the one below.