No astronaut has launched into space from American soil since the final flight of the space shuttle in 2011. Although the development of commercial space carriers has seen numerous successful launches of cargo, crewed human spaceflight remains the next major milestone.
When NASA's commercial crew development program began in 2009 as an intended replacement for the Space Shuttle, the first crewed flights were expected in 2015.
In 2014, NASA awarded commercial crew contracts to Boeing and SpaceX for their CST-100 and Dragon V2 capsule, respectively. At the time, former NASA administrator Charlie Bolden expected the US to end its reliance on Russian spacecraft by 2017.
Although the schedule has been pushed back since then, the latest target flight dates call for unmanned flight tests for SpaceX and Boeing in November 2017 and June 2018, respectively, followed by crewed launches in May and August 2018. Crew assignments for the first flights could come as early as summer 2017, about a year before launch.
Pressure to meet the deadline comes from NASA's move to not renew spaceflight contracts with Russia beyond the end of 2018. Other pressures are pushing the other direction - following a Falcon 9 explosion in September 2016, NASA has raised concerns about the safety of SpaceX's fueling protocol. Both companies announced scheduling delays as recently as December 2016.
Will the first commercial human spaceflight take place by September 2018?
This question will resolve as positive if a commercial space vehicle launched from the United States, carrying human passengers (even if test pilots), successfully travels to an altitude above 100 km and returns safely to the Earth, as reported by a credible news outlet, after 2010 and on or before August 31, 2018.
(Edit 1/6/18 for clarifications that test pilots/flights count, and that pre-2010 flights do not count.)