When did Juliet get married?

Juliet marries only once during the play.  Friar Lawrence marries Romeo and Juliet in Act 2, Scene 5, in the hopes of uniting the Montague and Capulet families.  Timing wise, the marriage takes place in the afternoon, the day after Romeo and Juliet meet at the Capulet party.  Of course, the marriage is initially kept a secret from most of the characters.

While Juliet is ultimately engaged to Paris, she does not go through with that marriage.  In fact, it is the "threat" of her impending marriage to Paris, coupled with Romeo's banishment to Mantua, that brings about the plan that will lead Juliet to initially fake her own death and, ultimately, to take her life.  It is only after she is dead that the majority of the characters (those that are still alive!) learn of her marriage to Romeo.