Falcon launches to the ISS typically have an instantaneous launch window (roughly, when the plane of the ISS's orbit sweeps over the launch site). If they miss it, they try again about a day later.

Falcon (both Heavy and its little sibling Falcon 9) uses subcooled densified propellants which limit how long it can sit waiting for launch while fueled before the LOX warms and expands too much.

A short window means they only have to compute and verify and program one trajectory.

You can get live countdowns and updates at https://www.spacex.com/launches which will link to first-party livestream coverage during the launch.

No need to go through third-party and advertising-laden dot-com sites.

The Falcon 9 and Falcon Heavy are intolerant to countdown holds late in the sequence. In particular, after propellant has been loaded, there must be no delay in launch. Any countdown holds will necessarily turn into a scrub for the day, as the propellant must be completely unloaded and the process restarted to prepare for launch. With the instantaneous window, it'll either be "go/no-go" at 07:26am EDT, or they'll go-around to another opportunity on Monday morning, around 07:22am.