In 1924, during a close approach between Earth and Mars, the US government coordinated a nationwide "National Radio Silence day" where radios were turned off for five minutes every hour. This was an early attempt to detect potential radio signals from Martians. Despite optimism, no signals were detected, as we now know there is no life on Mars.

Source: https://www.iflscience.com/in-1924-the-us-turned-off-their-radios-to-search-for-life-on-mars-73323