On what date did Louis XVI attempt to flee Paris during the Flight to Varennes?

Louis XVI attempted to flee Paris during the Flight to Varennes on June 20, 1791.

The royal family left the Tuileries Palace around midnight during the night of June 20–21. Louis XVI, Marie Antoinette, their children, and members of their household tried to travel east toward Montmédy, where the king hoped to find protection among loyal troops and regain greater freedom from revolutionary authorities.

The escape was badly organized and the large royal carriage attracted attention. The family was recognized at Sainte-Menehould, and the king was eventually stopped at Varennes-en-Argonne on June 21. They were arrested and escorted back to Paris, arriving on June 25.

The date can cause confusion because historians often describe the Flight to Varennes as occurring over June 20–21. The question asks when the attempt began, so June 20, 1791 is the standard answer. The failed flight badly damaged confidence in the constitutional monarchy: many revolutionaries concluded that Louis had betrayed the Revolution and could not sincerely support the new political order.