I give you a fair amount of background. Direct answer in bold at bottom. Enjoy!
Friar Laurence begins ACT IV with a visit from Paris. Paris is in the Friar's cell in order to tell him of Capulet's consent of the marriage between Paris and Juliet.
Poor poor pitiful (dense?) Paris never sees or refuses to see or doesn't care to see how Juliet never returns his love. However, when Juliet arrives to Friar Laurence's cell, Paris shovels his admiration towards her and Juliet coldly shields herself from it.
Once Paris leaves, Juliet is frantically emotional (surprise surprise) and is once again threatening suicide if things don't go her way (a violent temper tantrum). Friar comes up with a plan....
Friar's plan is this: Juliet will take an herbal concoction he knows how to mix that makes her appear stone cold dead for two and forty hours (42 hours boys and girls). She will take the potion and wake up dead (....) so her family will place her sheet wrapped body in the Capulet tomb. Meanwhile, Friar will send a message to Romeo to let him in on it, and eventually she will be reunited with Romeo and they will somehow live happily ever after.
My students typically ask at this point, "Why don't they just run off to Mantua together if they aren't going to be with their families?" A ha! I have no idea--the story would not be as tragic or intriguing.
Capulet changes his plan however! He moves the wedding from Thursday (two days away) to Wednesday (tomorrow!). This means that Juliet must take the potion that night and that it is unlikely that Friar's message will reach Romeo in time!