What are the witches planning at the start of the play Macbeth?

At the beginning of the play, in Act I, Scene I, the witches are planning a second meeting when the battle between King Duncan's men and the rebels is finished. The witches say that the battle will be over at sunset ("the set of sun").
As the witches continue their conversation, it becomes clear that they specifically intend to meet on a heath with Macbeth. The witches' knowledge of Macbeth's whereabouts helps to establish the theme of the supernatural early in the play.
In addition, at this stage in the play, the witches' reasons for meeting with Macbeth are not clear. However, by not revealing their motive for meeting with Macbeth, Shakespeare creates interest and suspense for the reader or audience member. In fact, the witches' intentions only become clear when they meet with Macbeth and Banquo in Act I, Scene III and reveal their prophecies.