The use of "evening" and "night" is often imprecise, but evening typically ends and night begins once the sun dips below the horizon. The precise moment may be defined as when the sun is no longer visible, or it may be when the sun's light entirely leaves the sky.
Evening is usually defined as the last few hours before sunset, when temperatures begin to fall from the heat of the day. Many regions use this term interchangeably with "dusk," referring to a very short period before sunset. In the Southern United States, however, it sometimes refers to a time period beginning somewhat earlier, even as early as mid-day.