A clear daytime sky can represent possibility, limitless potential, hope, or freedom.
A gray daytime sky can represent the dampening of optimistic emotions, or feelings of sadness, melancholy, irritability, grumpiness, wistfulness, reflection, etc.
A dark, threatening sky can represent a challenge or problem in your life, the feeling that a difficult period is coming soon, or "dark" emotions such as depression, anger, or despair.
A night sky can represent infinite possibilities, the magic of nature, among other meanings.