Today, SpaceX will hopefully launch and land a Falcon 9 rocket that it's already flown to space. The launch window opens at 2:10 PM and lasts for two hours; launch time is currently scheduled for 3:10 PM ET. You can livestream the launch, with commentary, at SpaceX's website.
This mission is called BulgariaSat-1 and will carry Bulgaria's first geostationary communications satellite into a high geostationary orbit around the Earth. It's launching from Kennedy Space Center in Florida, and a drone ship called "Of Course I Still Love You" will be waiting in the Atlantic Ocean for the Falcon 9's first-stage landing. If things don't go as planned, there's another launch window tomorrow at 2:10 PM ET.
This isn't the only SpaceX launch that's happening this weekend, though. On Sunday, a Falcon 9 will lift off from Vandenberg Air Force Base in California carrying 10 satellites for the company Iridium. Liftoff is set for 1:25 PM PT, and the company will once again attempt to land the first stage of the rocket.
It's not the first time SpaceX has flown and landed a flight-proven rocket; that happened on March 30. But rockets are the most expensive part of spaceflight; by reusing Falcon 9 first stages, SpaceX is cutting costs considerably. Reusable components are crucial to the future of human spaceflight, and SpaceX is certainly making steady and significant improvements in that regard.
Update, 6/23/2017, 1:05 PM: SpaceX tweeted that the ground team is taking additional time for checks. New liftoff time is 3:10 PM ET.