Dreaming of a bar mitzvah means you are transitioning out of childhood and into an adult. The bar mitzvah party is a sign that you are ready to take on more responsibilities in your life. No longer are you dependent on others to make decisions for you, but instead you will have to make tough decisions after thinking them through.
If you dreamt about a bar mitzvah that was joyous and successful, this could potentially be interpreted as a sign of your confidence and optimism about your ability to successfully navigate the challenges and transitions that are currently happening in your life.
If the bar mitzvah was not your own but that you were attending, this could potentially be interpreted as a reflection of your feelings about the relationships and connections that you have with the people in your life.