Why is Macbeth so concerned about where Banquo will be riding on the day of the banquet?

After hearing the latest of the Weird Sisters' prophecies, Macbeth is determined to kill Banquo and his son Fleance. The prophecy said that, though Banquo would never ascend the throne of Scotland, his heirs and successors would. Macbeth interprets this as a clear threat to his power. So long as Banquo and Fleance are alive, he will feel even less secure on the throne.
Having resolved to kill Banquo and his son, Macbeth needs to know their precise whereabouts on the day he intends to have them murdered. That's why he's so keen to find out where Banquo and Fleance will be riding that day, and how far they are intending to travel. Macbeth wants to make sure that his wicked murder plot goes according to plan. The last thing he wants is for Banquo and Fleance to get wind of what he has in store for them and flee the kingdom as Malcolm and Donalbain already have.