Although Lord Capulet moves the wedding a day forward, it would not have any effect on Friar Laurence's original plan. The plan was that Juliet would take the potion, which would induce a death-like sleep that same night (Tuesday) and she would awaken forty-two hours later. This would happen irrespective of whether the wedding to Paris was held on Wednesday or Thursday. Friar Laurence had sent word about this plan to Romeo, in exile at Mantua, via Friar John.
The plan was that Juliet, believed dead, would be interred in the family's burial-vault. Romeo would have gotten word of this plan and he and Friar Laurence would be in the vault the next morning, waiting for her to awaken. She would then accompany Romeo to Mantua.
"Shall Romeo by my letters know our drift
And hither shall he come: and he and I
Will watch thy waking, and that very night
Shall Romeo bear thee hence to Mantua."
Unfortunately, this was not to be. Friar John had been delayed since he and another Friar had been locked up in a house overnight by "searchers of the town" to prevent the spread of a suspected pestilence and he could not deliver the message to Romeo. He also could not find someone else to deliver the message and returned the letter to Friar Laurence.
It was this unfortunate and unforeseen event which led to the tragic denouement of the play: the suicides of both Romeo and Juliet. Romeo had received news of Juliet's 'death' and her burial. He then purchased a deadly poison from a poor apothecary which he drank on seeing Juliet's lifeless body in the vault. This happened after he had gotten into an altercation with Paris, killing him in a sword-fight. When Juliet awakens later and finds Romeo lying dead beside her, she takes his dagger and commits suicide, thus lending credence to the Prince's observation:
"For never was a story of more woe
Than this of Juliet and her Romeo."
In Act Four of Romeo and Juliet, Lord Capulet meets with Juliet after she has returned from "confessing" (in truth, planning her escape) in Friar Laurence's cell. He finds that her "peevish self-will'd harlotry" has disappeared and been replaced by a new commitment to obedience. After Juliet begs for his forgiveness, Lord Capulet decides that her new enthusiasm should merit a change in plans: the wedding is to be moved to the next day, rather than being held on Thursday as initially scheduled. On learning this news, Juliet asks her Nurse:
Nurse, will you go with me into my closet,
To help me sort such needful ornaments
As you think fit to furnish me to-morrow?
After retiring to her chambers, Juliet takes the drug that mimics death given to her by Friar Laurence, and she falls into a deep slumber.
The trouble with this change of plans? It's all happening too fast! The original plan created by Friar Laurence and Juliet just hours earlier was contingent on the wedding happening on Thursday; this would have allowed an appropriate amount of time for Friar Laurence's messenger to make it to the place of Romeo's banishment, deliver him the news of Juliet's fake death, and order Romeo to meet her in the Capulet crypt.
Instead, the messenger is left with an incredibly small window of opportunity to get this message to the boy--less than twenty-four hours! He gets boarded up in a house that has been hit with the plague and is unable to deliver the message on time.
For this reason, Romeo learns from Benvolio that Juliet is actually dead. This miscommunication drives Romeo to buy poison from an apothecary and rush to the crypt, where he kills himself at Juliet's side. Awakening, Juliet sees her dead husband and throws herself upon his dagger.
Thus, we could argue that this simple scheduling mishap is a major factor in Romeo and Juliet's tragic deaths.
Lord Capulet, Juliet's father, sets the wedding up from Thursday to Wednesday after Juliet consents to marry Paris. He gets excited that she is "finally" coming around to what he wants that he isn't going to waste any more time and risk the chance that Juliet might change her mind again; so, he exclaims, "Send for the County; (Paris) go tell him of this. I'll have thise knot knit up tomorrow morning (IV.iii.23-24). This could upset the Friar's arrangements with Juliet because of two major things: first, there's not enough time to let the Friar know that the faking of her death is happening right away; and second, because of that lack of time, the Friar's time to get a letter to Romeo about the new plan is limited by 24 fewer hours.