.
The moon is being drawn to the earth by dark power, where it will eventually collide into it in. As Link, you are given a time traveling shapeshifting instrument (that can do other songs as well), and three days to make everything right. Some items you can carry with you when you travel, but many you cannot. Some events require it to be a certain hour or day to encounter, which makes the game fun to mentally map out.
Edit: Right, and the masks are a fun mechanic.