Ever since the Space Shuttle program ended in 2011, NASA has relied on Russian Soyuz missions to get astronauts back and forth from the International Space Station. In order to maintain control over its own missions, the Commercial Crew program has sought replacements, with SpaceX and Boeing in competition for the contract.
After a number of delays, SpaceX will go first with a test launch of its Crew Dragon vehicle, scheduled to take off from Launch Complex 39A in Cape Canaveral at 2:49 AM ET. SpaceX will have a live webcast of the Crew Demo-1 launch, which you can watch here, along with coverage from NASA TV. The broadcasts should start at 2 AM. To synchronize with the ISS, it's an instantaneous window so there's no waiting for this one -- it either will or will not go tonight.