Macbeth refers to the day as "foul" because so many people have died in the battle against the Norwegians and Scottish rebels and "fair" because the Scottish soldiers won the battle. He grieves for the loss of life but rejoices in victory.
In the opening scene of the play, the witches use the same words when they say, "Fair is foul and foul is fair," and set the tone for the play. The audience has heard their words, but Macbeth doesn't know about them, thus fulfilling the definition of dramatic irony. The confusion between what is good and what is evil will pervade the play, particularly for Macbeth.
Or he's commenting on the weather "the fog" and delighting in the massacre. The coordinating pronoun "and" aligns "foul" and "fair"--and Macbeth's starting with "foul" reveals the first image in his mind. Macbeth is "bloody," an exceutioner. He split a guy open from the groin to the mouth and stuck his head up on a stick on the field, and he wasn't even bothered at all to go a second round. Though the king is shocked and asks "Dismay'd not this / Our captains, Macbeth and Banquo?" the Sergeant even jokes about how killing is in Macbeth's nature: "As sparrows eagles or the hare the lion" (as sparrows shock eagles or hares shock lions).
A dramatic irony is also that the audience knows that he will fall--from the play's title of course, but also from the "I have not seen"--standard foreshadowing language, still used in horror movies today--by tying the foul and fair to the "I," the hero can see external nature but not internal corruption--or even better--that he thinks he sees corruption but doesn't, creating dramatic irony upon irony with "I have not seen"--and so it just hit me--he's starting to "make assurance double sure.' So his first words and his blindness to their meaning reveal his nature.