NASA on Monday postponed for at least two days Saturday's planned launch of the shuttle Discovery because of failure of a system that separates the booster rockets and fuel tank from the orbiter. The liftoff of the shuttle with a crew of five astronauts was rescheduled for no earlier than March 13. NASA spokesman Karl Kristofferson said the trouble is with a master events controller, one of two such shuttle systems that receive separation commands from the spaceship's main computer. The problem showed up early Monday during a routine test. The controllers arm and fire explosive devices that separate the two solid fuel rockets two minutes after liftoff and the external fuel tank when the main engines burn out about 81/2 minutes into a flight.