Why does Caesar mention his desire that Calpurnia stand "directly in Antony's way"?
The line you’re referring to happens in Act I, Scene 2.
Here’s the situation: The Romans are attending and participating in a pagan religious festival called Lupercal. This festival includes a race in which the character Mark Antony is running. Antony is personally close to Caesar. Caesar’s wife, Calphurnia, is barren.
There is a superstition that when one of the runners in the race touches a barren woman, she “shakes off her sterile curse.” So Caesar says to Calphurnia:
Stand you directly in Antonius' way, When he doth run his course.
Calphurnia does not become pregnant however. But this scene foreshadows the fact that Caesar and Antony have a bond, and this will come into play in an important way later in the drama. This is just one example of superstition in the play.
There are several others, including telling the future by reading the entrails of a bird.