Our Lord appears in the middle of a dance to Maria Faustina to call her attention to her call to a life of holiness

Saint Faustina from a very young saint felt in her soul a call from God to a religious life of dedication to Him. But her lack of understanding led her not to heed this call, inding in her youth to a life of amusement of creatures in the illusory attempt to drown out that voice. In response he could not get rid of the torment in his soul of the call and was not content with the satisfactions the world offered him.

One day he went to a dance with his sisters. Everyone had a great time. He tried to go dancing to forget the torment that afflicted him at that moment again. But... Suddenly he saw nothing else around him, disappearing to the music:

Jesus then appears beside Faustina, all covered in wounds, and says to him, "How long shall I be patient with you and even when will you let me down?"

Faustina leaves the ball at the same moment and runs to the Cathedral Estanislau Kotska and pours out in front of the Blessed Sacrament crying out for directions to fulfill his Holy Will. God through his inner voice makes several directions to her from that moment on to lead him to a congregation. 

The inner locutions of Jesus to Saint Faustina will never cease and receive visits from Our Lady with the Child Jesus while she remained in the congregation. But she was always transferred. Jesus asked her to report everything to her superiors, who repeatedly tried to induce her that she was having illusions, that only saints had this relationship with God and not a sinner like her, leading her to believe it a few times.

But in one of these assaults of doubt whether this inner voice came from the Father or not, he asked God in thought: "If you are truly my God who ye remain with me and speak to me, that this education will confess even today, and this sign will strengthen me" 

But she didn't need a day, because at that very moment she asked her to confess before her eyes! Then Jesus said to him inside, "Do you believe me now?"

Her soul strengthened after this event and she was greatly amazed of herself having a doubt like this, even for a brief moment.

Reference:

Diary of Saint Faustina: Divine Mercy in my soul