From Westchester.gov:

Entergy will conduct a full volume test of the new Indian Point sirens on Thursday, August 14 between 3:30 p.m. and 4 p.m.  All the sirens in the new system will be sounded for four minutes at full volume. This is only a test. No response on the part of the public is necessary.

Westchester County officials will use the current (old) siren system to alert the public during an emergency at Indian Point until the new siren system accepted and placed in service.

In an actual emergency, all the current (old) sirens would sound at full volume for four minutes, followed by the broadcast of an Emergency Alert System (EAS) message on radio and television stations. Sirens are not a signal to evacuate; but rather for people to tune to an EAS radio or TV station for information and instruction.

Once again the new siren system sounding on Thursday at 3:30 is only a test. No action on the part of the public is necessary.