This paradox of foul/fair by Macbeth alludes to the prediction of the weird [meaning "destiny-serving] sisters at the beginning of the play: "Fair is foul, and foul is fair" (I,ii,10). In uttering much the same words, Macbeth senses the force of fate in his life, but he is perplexed by the witches' referring to him as Thane of Cawdor and King:
...I know I am Thane of glamis;/But how of Cawdor? The Thane of Cawdor lives,/A propsperous gentleman; and to be King/Stands not within the pospect of belief,/No more than to be Cawdor. (I,iii,71-75)
Still, this idea is planted in Macbeth's mind, and he demands that the witches tell him how they have such information. Later, Banquo cautions Macbeth against giving credibility to what the witches have told him:
And oftentimes, to win us to our harm,/The instruments of darkness tell us truths,/Win us with honest trifles, to betray 's/In deepest consequence. (I,iii,123-126)
By not heeding Banquo's advice, Macbeth considers the prophecy of the weird sisters as "fair," and seeks the throne of Scotland. However, in so doing, he commits the "foul" acts of regicide as well as other murders. Clearly, for Macbeth "fair is foul," and "foul is fair" as justice is served him in the end. Having ignored his premonitions on the fair/foul day on which he encounters the weird sisters, Macbeth brings the prediction of the evil sisters to fruition. Clearly, Macbeth's line, "so foul and fair a day I have not seen" (I,iii,38) foreshadows his fateful end in Shakespeare's great tragedy, "Macbeth."
As this is Macbeth's opening line, it could have several different connotations.
"Foul" could refer to the weather on the heath, a stormy day. It could also refer to the battle itself, in which many good men died.
"Fair" is in reference to the victory they have gained. They have put down the rebellion and have established Duncan more firmly on the throne.
This also foreshadows Macbeth's future. "Fair" presages his own rise to the throne, as well as the promise of Banquo's own sons also gaining the ascendency.
"Foul" will of course refer to the tragedy that is to come. Duncan dies, as well as Macbeth and his wife.
you must take the modern english version of this line as follows.
"Macbeth feels that the day had been an admixture of good and bad. The good and happy event is the one that they have fought against a formidable enemy and emerged victorious and returning home feeling elated.
The bad been the bitter weather that had added and made more pronounced their tiredness."