This is a test. This is only a test. But it's going to be one very big test.

On Wednesday, October 4 at 2:20 p.m. ET, every TV, radio and cellphone in the United States should blare out the distinctive, jarring electronic warning tone of an emergency alert, accompanied by a notice along these lines:

“This is a nationwide test of the Emergency Alert System, issued by the Federal Emergency Management Agency, covering the United States from 14:20 to 14:50 hours ET. This is only a test. No action is required by the public."

Here's what to know:

What's going to happen?

On Wednesday, October 4, a test is planned of the entire nation’s Emergency Alert system, a tryout to ensure everything is working correctly in the event of a big, national disaster or attack.

In the wake of the horrific fires on the island of Maui on August 8, when warning sirens that might have alerted people to the danger weren’t deployed, it’s a reminder of what systems are in place should they be needed.

Why is a national test necessary?

Federal emergency management coordinators need to make sure the national alert system is still an effective way to warn Americans about emergencies, natural catastrophes, attacks and accidents at the national level.

What will the emergency message say?

The exact wording hasn't been released yet but it's very likely to be something along these lines: “This is a nationwide test of the Emergency Alert System, issued by the Federal Emergency Management Agency, covering the United States from 14:20 to 14:50 hours ET. This is only a test. No action is required by the public."

On cellphones, it will come as a text message:

“THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”

Phones on which the main menu set to Spanish will see this: “ESTA ES UNA PRUEBA del Sistema Nacional de Alerta de Emergencia. No se necesita acción.”

When will it happen?

On Wednesday, October 4, the message will go out at the same moment across every time zone in the United States.

That means 2:20 in the afternoon in the East, 1:20 p.m. Central time, 12:20 p.m. Mountain time and 11:20 a.m. on the West Coast. People in Alaska will hear it at 10:20 a.m. and in Hawaii the alarms will go off at 8:20 a.m.

How long will the test last?

The test is scheduled to last approximately one minute. It will only go out once, there will be no repeats.

Where will it be heard and seen?

The message will be heard and seen pretty much everywhere. It's being conducted with the participation of radio and television broadcasters, cable systems, satellite radio and television providers and wireline video providers.

So all across the United States, TV shows will be interrupted, radio programming halted and phones will get a warning message. The message will go out in both English and Spanish, showing up most places in English but in Spanish depending on the language settings of the device.

Has something like this been done before?

The first national emergency broadcasting system in the U.S. was created in 1951 as a way for the government to use radio networks to warn the nation of an enemy attack during the Cold War. It was further refined and expanded as fears of nuclear attack grew in the 1950s and 60s.

The first nationwide test of the most recent version of the Emergency Alert System took place on November 9, 2011, at 2:00 p.m. ET.

That check revealed multiple problems with the national Emergency Alert System that could have meant not everyone would hear the alert in the event of a real emergency. With that knowledge, the system was improved and strengthened.

That 2011 test message was a lot wordier than those used now.

"This is a test of the Emergency Alert System. This is only a test. The message you are hearing is part of a nationwide live code test of Emergency Alert System capabilities. This test message has been initiated by national alert and warning authorities. In coordination with Emergency Alert System participants, including broadcast, cable, satellite, and wire line participants in your area. Had this been an actual emergency, the attention signal you just heard would have been followed by emergency information, news, or instructions. This is only a test. We now return you to regular programming."

The test on Oct. 4 will be the seventh nationwide test sent to radios and televisions, the third to consumer cellphones and the second to all cellular devices.

Will there be warnings about the test?

Expect a tidal wave of news stories and warnings leading up to the test, to avoid panic.

FEMA and the FCC are coordinating with wireless providers, television and radio broadcasters, emergency managers and others to get the word out. The goal is to minimize confusion and maximize the public safety value of the test.

Could anything stop the test?

It could be postponed if there is "widespread severe weather or other significant events," according to the FCC. The backup testing date is set for Oct. 11.

Are there ever false alarms?

Over the years there have been mistaken messages sent out that triggered false alarms at the local level, especially in the 1950s when the system was new and communication more difficult.

The most recent false alarm occurred in 2018 in Hawaii when the Hawaii Emergency Management Agency system mistakenly sent an alert notification warning of a ballistic missile threat to the Islands. During a shift change, someone had selected the wrong item on a computer.

A flurry of tweets, often with screenshots of the message, popped up on cellphones shortly after 8 a.m. local time. The message read, "Ballistic Missile Threat Inbound To Hawaii. Seek Immediate Shelter. This Is Not A Drill." Some state highway signs also noted the warning.

It took 38 minutes to clarify that the alert was due to user error.