Why is Macbeth asking Banquo questions about his plans for the afternoon in Macbeth?

Macbeth is planning to kill Banquo and wants to make sure he knows where to send the murderers. 

Macbeth is pretty convinced that Banquo is on to him.  Banquo knows as much about the witches’ prophecies as Macbeth does.  He was there when the witches told him he would be king.  He was also there when Duncan was murdered.  Macbeth fears that Banquo is suspicious.  He is. 

In a soliloquy, Banquo tells the audience that he is afraid that Macbeth got his kingship through evil means.  When he says “Thou hast it now” he is saying, “Macebth, you are king.”  He is not talking directly to Macbeth, of course.  He also says that Macbeth “play'dst most foully for't,” meaning he got to be king through despicable acts, in this case killing the king (Act 3, Scene 1). 

Banquo knows that Macbeth is dangerous though, and he does not let on.  When Macbeth tells him he wants to see him at the ball, Banquo pretends he has no suspicions and is ever-loyal.  He worries about what Macbeth would do if he thought Banquo knew what he had done. 

MACBETH

To-night we hold a solemn supper sir,
And I'll request your presence.

BANQUO

Let your highness
Command upon me; to the which my duties
Are with a most indissoluble tie
For ever knit. (Act 3, Scene 1) 

Macbeth casually asks Banquo if he is riding, and how far, and reminds him not to miss the feast.   Banquo complies, but he is wary.  Later, when he sees the murderers Macbeth has sent, he knows what is going on.  He is with his son Fleance, and he tells him to run.  Fleance is able to get away then, but Banquo is not that lucky. 

Later, Banquo does make the banquet.  He comes as a ghost!  Macbeth is horrified when he sees him, and while Lady Macbeth makes excuses for him the guests are still disturbed by his actions.  It is a sign of Macbeth’s guilt and mental decline.