There will be a test of the Indian Point sirens in Westchester, Rockland, Putnam and Orange Counties on Sept. 21 at approximately 6 p.m. This sounding is only a test.
If there were an emergency at the nuclear power plant, the sirens in the Emergency Planning Zone (EPZ) would be sounded continuously at full volume for four minutes, followed by an activation of the Emergency Alert System (EAS) on radio and television stations to broadcast important information and instructions. The sirens are not a signal to evacuate, rather they are intended to alert the public to tune to an EAS radio or TV station for important information.
This sounding is ONLY A TEST. No action by the public is necessary.

Per Westchestergov.com