In the ancient religion of Vtudian, which I, possibly, completely made up, the reason given for night and day is simple. The sun and the moon are an old married couple. Like most corporeal beings, when they first fell in love, they shared a bed, and spent a great deal of time together. So, while there were occasional moments of night time where you couldn't see the sun, and a few moments of day time, when you couldn't see the moon, often they'd share the sky in a perpetual duskish.
But as he got older (because despite what Marge Piercy tells you, the moon isn't Always Female in myths that I may be lying about existing), the moon developed a wicked case of morning breath, and the sun drifted away from him. Oh, sure, sometimes you can see the moon when the sun is coming up or down, but they tend to sleep alone and keep mostly to themselves.
Can you come up with a cure for the moon's perpetual morning breath? Or find some other way to get these two back together?