http://www.newsweek.com/nasa-sls-wet-dress-rehearsal-postponed-spacex-axiom-1-launch-mega-moon-rocket-1695515

A crucial test of NASA's Space Launch System (SLS) rocket has been postponed until after a SpaceX launch later this week.

The so-called wet dress rehearsal of SLS is an important test that involves powering up parts of the rocket and launch facilities before pumping the rocket full of fuel—hence the 'wet' part of the name—and counting down for launch. The countdown will then be cancelled shortly before a real launch would take place.