In Act 3, Scene 1, Banquo tells Macbeth he is going riding. Note how Macbeth asks casual, seemingly innocent and friendly questions to find out what he wants to know. "Ride you this afternoon?" (line 21). "Is't far you ride?" (line 26). "Goes Fleance with you?" (line 39). Banquo tells Macbeth that he won't return until after dark (3.1.27-30). This is all the information Macbeth needs to set up an ambush. The murderers attack Banquo and his son when they are returning that night. We know it is nighttime because Banquo and Fleance enter with a torch. Although they have been riding, they are now on foot. Shakespeare could not bring a pair of horses on the stage, so he has the Third Murderer explain to the First Murderer that Banquo and his son have had to walk the last mile from where they left their horses.
Almost a mile, but he does usually [i.e. walk]
(So all men do) from hence to th' palace gate
Make it their walk. (3.3.17-19)
Shakespeare arranges to have Banquo and Fleance return in the dark because that makes it easier for the audience to accept the fact that the young boy could escape three murderers.