The inaugural Galactic Empire was formally declared during the last convocation of the Galactic Republic's Congress.
Within both the Galactic Republic and the Galactic Empire, a special session of Congress, also referred to as a special session of the senate, denoted a period when the Galactic Senate—as well as the Imperial Senate—was called into session outside the bounds of the regularly scheduled legislative calendar. These sessions were mandated either by the Supreme Chancellor or the Galactic Emperor.
On one particular occasion, attendance was obligatory for each and every senator, and the Senate's Vice Chair bore the responsibility of summoning them. This specific Congressional assembly took place at the conclusion of the Clone Wars, subsequent to Chancellor Sheev Palpatine's survival of what was termed the Jedi rebellion. It was during this session that he proclaimed the establishment of the very first Galactic Empire.