In Shakespeare's day, "husbandry" meant the skillful running of a household. The term also implies thrift in the day-to-day management of the domestic economy, and that's how Banquo uses it in these lines. When he says that "There's husbandry in heaven; the lights are all out," he means that the angels in heaven are illuminating only a handful of stars in order to economize. This scene foreshadows the gloom that will descend upon Scotland once Macbeth has usurped the throne. The hanging sky, darkened by the angels' stinginess, provides an appropriate backdrop for the evil that is about to take place at Macbeth's castle.
The enveloping darkness in this scene, then, is as metaphorical as it is literal. This makes it easier for an audience during an open-air performance to enter into the spirit of the scene without paying too much attention to the bright sunshine that may be blazing overhead.
You raise an interesting question. How can Banquo claim to be looking up at the night sky when the play is being performed in broad daylight? Shakespeare seemed to care little about authenticity in stage settings. The opening lines of this scene are intended to establish that it is after midnight, and the audience is supposed to imagine it as such. The torch that Fleance is carrying is the only prop. Shakespeare frequently used dialogue between characters to convey information to his audience. Even today Shakespeare’s plays are often performed in the open air in public parks in the day time. If this scene from Macbeth were being performed in daylight in such a place, the audience would just have to understand that it was nighttime.
A lot happens in this very short scene. Banquo confesses that he cannot sleep because he is tormented with thoughts which must have been evoked by the meeting with the Weird Sisters. Time is of the essence. It has to be nighttime because the King must be asleep. Banquo meets Macbeth and they talk in hushed voices in guarded language. Macbeth subtly sounds Banquo out about participating in a bloody coup. Banquo turns him down flatly, saying, “So I lose none [honor] / In seeking to augment it, but still keep / My bosom franchised and allegiance clear, / I shall be counseled.”
Macbeth has to commit the murder alone. The scene ends with him headed for Duncan’s chamber with a dagger. No doubt he would have preferred to kill Duncan’s two sons the same night but thought he needed help. All this happens in just a few pages. It must be in the dead of night when everyone else is asleep.