Macbeth's words "So foul and fair a day I have not seen" express both the bellicose and bizarre happenings of the day as well as the unpropitious and victorious occurrences of Act I of Shakespeare's play Macbeth.
Ostensibly a foul day, Macbeth later emerges from a bloody battle with the traitorous Macdonwald as the "fair" victor and is named Thane of Cawdor. After this, he enters a heath and sees the "weird sisters," who are menacing, and who seem to blur the lines between good and bad, making "nothing what is not"—that is, both "fair" and "foul." Then, they predict that he will become king. And, although these new honors seem wonderful, they do not sit well with Macbeth and he wants to think more about them.
Later in Scene 5, Lady Macbeth changes the good news of Macbeth's elevation to Thane of Cawdor to concern about Macbeth's becoming king and his lack of desire to go after the crown. Further, she urges him to kill King Duncan while he is at their castle. This urging of hers puts Macbeth into a brown study and he becomes lost in the dark thoughts of his "vaulting ambition" in a soliloquy in which he vacillates between "fair" and "foul" thoughts on the goodness of Duncan who is a kinsman and a king who has been blameless in his office in contrast to Macbeth's own "vaulting ambition which o'erleaps itself."
Finally, in the last scene after he has committed "this terrible feat" of regicide, Macbeth becomes more foul as he vows that "False face must hide what the false heart doth know" so that the "foul" acts of his will seem "fair."