Time

Most Minecraft life orbits around the time of day. During the day, gameplay is much different than in the night. For instance; during the night, the light level is low enough that hostile mobs can spawn.

Day/Night Cycle
Minecraft has a day/night cycle, with varying game play during each part. The cycle begins with day (ten minutes long), and shifts to night during the sunset. Night is seven minutes long, and transfers to day via sunrise.

Day
During the day the outside light level is at its maximum; fifteen. This allowing passive mobs to spawn anywhere outside, and also causes hostile mobs to burn; making it safe to leave shelter.

Sunset
Sunset is the what transforms day to night. It slowly changes the light level from fifteen to four.

Night
In the night, the hostile mobs come out, due to the low light level of four.

Sunrise
Sunrise marks the demise of most hostile mobs (with the exception of creepers), due to it bringing the light level from four to fifteen.