The Operation Doomsday is the name given by Saturn to the attack launched by him on the main headquarters of the Maverick Hunters on June 19th of 2193, as shown in Prologue: Doomsday. Saturn and Shad sneaked into a sealed hangar where the Reploid colossus Gigantex was hidden and used his mysterious power to reactivate him and send them through a vortex to the Maverick Hunter HQ.
X and Zero were outside the base chasing a trio of identical Mavericks, and soon before Saturn's move an extremely jamming device was activated near the headquarters, cutting the communication between the Hunters and their Navigators and preventing any safe teleports around the area, forcing X and Zero to return with a Ride Chaser and arrive too late to stop the destruction.
The Operation resulted in the destruction of most the main headquarters, the deaths of several Maverick Hunters, the disappearance of the Commander Signas, the kidnapping of Axl, and severe damage to survivors such as Layer and Pallette. Saturn would later use that event as a proof of his power to manipulate Mavericks into joining his rebellion and begin the Great Neo Maverick War. It also put the world in panic and allowed councilor Raffould to convince the rest of Earth's Government to approve his project of creating the Earth Army.