During daylight, the sky appears to be blue because air scatters

more blue sunlight than red. At night, the sky appears to be a mostly dark surface or region spangled with stars. During the day, the Sun, and sometimes the Moon, can be seen in the sky unless obscured by clouds. In the night sky the Moon, planets, and stars can be similarly visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation can also be seen in the sky. Birds, insects, aircraft, and kites fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.

Source: Wikipedia