In Act III, why does Macbeth invoke the night?
I'm gathering you're talking about Act 3 Scene 2:
Come, seeling night,
Scarf up the tender eye of pitiful day;
And with thy bloody and invisible hand
Cancel and tear to pieces that great bond
Which keeps me pale!--Light thickens; and the crow
Makes wing to the rooky wood:
Good things of day begin to droop and drowse;
Whiles night's black agents to their preys do rouse.--
Thou marvell'st at my words: but hold thee still;
Things bad begun make strong themselves by ill:
Night represents moral darkness, the spirits of darkness, and concealment. Macbeth is preparing to have Banquo murdered, and, just as Lady Macbeth earlier wished for the spirits to "unsex" her, Macbeth wishes for night to cover any possibility of pity (scarf up the tender night of pitiful day) and to give him the concealment necessary to do his deed without being caught. There's power in the darkness, both literally, and, for Macbeth, figuratively. By now he has completely crossed the line and he no longer wants to be in the sun.
It is in Act III, Scene II that Macbeth invokes the night through the words, "Come, seeling night." To put this into context, Macbeth is about to send his henchmen to kill Banquo and his son, Fleance, and he begs the night to come so that its darkness might conceal these murders from others ("blindfold the kindhearted day").
As Macbeth continues, he urges the night to use its "bloody and invisible" hand to commit these murders and so bring about an end to Macbeth's "fear." Remember that Macbeth is afraid of the prophecy in which it was foretold that Banquo's sons would be the kings of Scotland. As such, Macbeth wants to protect his crown and is prepared to go to any lengths to ensure that Banquo's sons do not take it from him.
In the next lines, Macbeth likens himself to a predator who awakens in the night to hunt his prey, represented here by Banquo and Fleance. This is significant because he is suggesting that these murders are legitimate--that they are an inevitable part of the natural order and that they cannot be avoided.