The dagger Juliet threatens to kill herself with makes three appearances; it appears: 1) with Friar Laurence if he cannot think of a way to end her betrothal to Paris; 2) with her alone in her room just before she drinks the vial; and 3) with her in the tomb. In all three appearances the symbolism is pretty much the same, though there are other specific connotations. It is culturally recognized that because the dagger is a mini sword that can easily be concealed, it is a symbol of deceit and corruption ("What are Daggers?," knife-depot.com). Although, we do see other connotations per scene.
It is not insignificant that Juliet draws out the dagger in front of Friar Laurence and threatens her life with it as a remedy to prevent her from marrying Paris. In this scene, Act 4, Scene 1, it is very clear that Juliet is very close to committing one of two sins. If she were to marry Paris, she would be committing a sin by entering a polygamus marriage. Likewise, Friar Laurence would be committing a sin by knowingly joining two people in a polygamus marriage. Hence, not only does the dagger symbolize Juliet's deceitful marriage to Romeo and Friar Laurence's deceitful plan, it also symbolizes both Juliet's and Friar Laurence's corrupt break from their religion.
In Act 4, Scene 3, Juliet picks up the dagger again while worrying about whether or not the potion will work. She asks herself:
What if this mixture do not work at all?
Shall I be married then to-morrow morning?
No, no: this shall forbit it: lie thou there. (21-23)
Since she is willing to use the dagger to prevent her marriage to Paris, the dagger also symbolizes her frailty as a woman. She is trapped by her father's command to marry Paris, and one of her only means of rescuing herself is by taking her life with a mini sword, rather than combating the situation with a real sword, as a man would do.
Finally, Juliet uses the dagger to kill herself in her tomb after she sees Romeo lying there dead. At this point, the dagger represents the fraud she has committed in secretly marrying Romeo and faking her death. But it also represents the treachory associated with the families' feud, which is ultimately to blame for both her faked death and her real death.