The English name for Monday comes from the Anglo-Saxon word Mōnandæg, which loosely means “the moon’s day.” Mōna is the word for moon in Old English. The second day of the week has been classified as the moon’s day since Babylonian times. The Babylonians were the ones who decided on a seven-day week, and they named five of the days for planets, and one each for the sun and for the moon. Ancient Romans followed the same pattern, though they technically thought they were naming every day after a planet, since the Romans thought that the sun and moon were planets in their own right.

Source: Dictionary.com