Have you ever been outside very early in the morning, when it's still completely dark outside, waiting for the sun to rise? Then, you see that sliver of light on the edge of the horizon, like someone has opened a small crack in a door? That would be The Crack of Dawn.
A similar phrase, The Crack of Day, means the same thing. As I've mentioned before, getting up early is not something I generally do. I'm more of a sunset girl, myself, but I have seen my share of sunrises. They can be a beautiful thing.