An Extraordinary Session, also known as a Grand Convocation, a special session of Congress, or a special session of the senate, was a special meeting of the Galactic Senate of the Galactic Republic called by the Supreme Chancellor to which every senator was obliged to attend.
Supreme Chancellor Palpatine, secretly the Sith Lord known as Darth Sidious, called an Extraordinary Session to issue his Declaration of a New Order and begin the Galactic Empire. This Extraordinary Session was held in the Grand Convocation Chamber on Coruscant. Vice Chair Mas Amedda summoned Bail Prestor Organa to the session. Palatine stated that the Jedi had tried to overthrow the Republic, and Palpatine became the Emperor of his new empire, allowing the Sith Lord to control the galaxy as the Grand Plan of the Order of the Sith Lords planned for. Emperor Palpatine would rule the galaxy for years, but was ultimately killed by his former Sith apprentice, Darth Vader, now redeemed to be Anakin Skywalker as he had been before falling to the dark side of the Force, on Death Star II during the Battle of Endor.