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Update: The countdown and static fire test has been completed, apparently successfully. The launch however is now set for Thursday.
SpaceX today plans to do the first static fire dress rehearsal countdown of a Falcon 9 rocket using an already used first stage.
The static fire process for SpaceX and its Falcon 9 rocket is one of the last critical components in the pre-launch flow ahead of liftoff. For SES-10, the Falcon 9 and mated second stage will be moved to the launch pad on top of the TEL (Transporter Erector Launcher) and will be taken to vertical at historic launch complex 39A. Once the Falcon 9 is vertical, technicians and engineers will complete all of the connections between the TEL/launch mount and LC-39A and proceed into countdown operations on Monday morning.
For this particular static fire, SpaceX has up to an eight-hour window.
If all goes well, they plan to do the actual launch on Wednesday.
The article by the way also provides a nice detailed history of the first stage.