In the previous scenes, Juliet went against her father's wishes of marrying Paris.  What he does not know is that she is already married to Romeo.  So when he told her to marry Paris and she refused (at this point Romeo is banished and the two of them are emotionally a mess trying to figure out how to be together) her father was furious.  She is a child.  She is to do what he says and when he says to do it.  In this scene (Act IV, sc 2) she has already decided to take the potion, so she pretends to do as her father wishes.  When she asks for his forgiveness, he is ecstatic.  In fact, he decides because it's all so wonderful that his plan is working out, that they will have the wedding a day earlier (Wednesday).

"JULIET

Where I have learn'd me to repent the sin
Of disobedient opposition
To you and your behests, and am enjoin'd
By holy Laurence to fall prostrate here,
And beg your pardon: pardon, I beseech you!
Henceforward I am ever ruled by you.

CAPULET

Send for the county; go tell him of this:
I'll have this knot knit up to-morrow morning."