Let's first think about Light and Dark, essentially good and evil, but we also have to consider the geography and climate when it comes to these motifs. Think about Macbeth and how he goes about his business or when crucial moments come about. The day is dark when he first meets the "weird sisters" in Act 1: "So foul and fair a day I have not seen" (I.3.38). The day is "fair" since he has survived his battle against the Norweyans, but it is "foul" in that the weather is dark, the "thunder, lightening...rain" the witches refer to in Scene 1. Macbeth also kills Duncan in the middle of the night; he has Banquo killed by the three murderers at night (Banquo and Fleance need a torch as Banquo calls for "a light").
Macbeth (and Lady Macbeth) must put up appearances, a facade and false and loyal nature, in broad daylight, in times when everyone notices. They must pretend to be innocent when in public (the light), when at a dinner or banquet or receiving the King in their home. However, it is at times of night or foul weather (darkness) when their true colors, the real Macbeths appear. They are sinister and murderous and dangerous; or it is these moments of darkness when they receive information that sets them down an evil path.
In the light of day, they appear as a stable and noble and virtuous couple; by night, their real ambitions and treachery manifest themselves. Later, the novelist Robert Louis Stevenson would write The Strange Case of Dr. Jekyll and Mr. Hyde, truly exploring this idea of light and dark in man.