It is significant that Macbeth’s first words echo the witches’ comments about foul being fair and fair being fowl. Basically, what’s good is bad and what’s bad is good. There are two meanings to this. First, it means that things are not what they seem. Second, it means that things are two ways at once—good and bad. This is a long-running theme in the play. Things are not always good or bad. Sometimes they are both at once, or changing.
When Macbeth says, “So foul and fair a day I have not seen,” he is commenting on the effect the Weird Sisters have had on the landscape. They have clearly purposed to meet Macbeth, and they are intending to interfere with him. Even before the witches appear, there is a darkness in the air. When Banquo sees the witches, he is clearly surprised.
What are these(40)
So wither'd, and so wild in their attire,
That look not like the inhabitants o’ the earth,
And yet are on't? (Act 1, Scene 3)
This reference to the unearthly nature of the witches’ appearance reminds us that there are going to be some abnormal things happening in this play. Not only is something dark about to happen, but the entire play is shrouded in darkness.