The Promised Day refers to the coming time of the long-awaited culmination of Father's four-hundred year plan involving Amestris, the Human Sacrifices, the Philosopher's Stone and the Nationwide Transmutation Circle. It was first mentioned by Father during his first appearance in the manga as a toast to his faithful children before quaffing Greed's Philosopher Stone heart back into himself and has since been mentioned sparingly and with few details. It appears, due to his excitement regarding its proximity, that General Raven and the other members of Military High Command are well aware of the time of The Promised Day, though they may not know all of the true details. From the scattered hints given by those responsible, it would seem that the completion of Sloth's underground tunnel is essential to the Promised Day, but when Alphonse Elric questions his father Van Hohenheim on what their next move should be regarding the tunnel in Chapter 82, Van responds by telling his son to look not only at the ground, but also at the sky - suggesting that Hohenheim himself knows all the details regarding Father's true plan. In Chapter 83, Ling - knowing the truth from inside Greed's soul - relays to Edward that Father plans to open The Gate when the Promised Day arrives.
Al and Van, knowing the truth, begin a contact chain from Mason to Sig and Izumi Curtis to Major Miles and Captain Buccaneer to General Grumman to 2nd Lt. Rebecca Catalina to Lt. Riza Hawkeye to Jean Havoc to Colonel Mustang, informing them all that the Promised Day - the end of this great battle - will take place in the Spring of 1915. The manga takes a short time skip to Spring 1915, when both sides gather all their resources for the final clash. In Chapter 89, as the final march begins, the younger siblings of Sergeant Denny Brosh declare that there is to be a solar eclipse on that very day, suggesting perhaps (in conjunction with Hohenheim's earlier statement) that the eclipse (and perhaps the celestial circle created by the sun's corona around the moon) is part of the ritual that will open the Gate. Though it isn't suggested in the manga or the anime, some speculate that the Promised Day occurs on the equinox. The reason this is speculated is because the equinox is considered holy by many ancient civilizations.
Despite the combined efforts of everyone involved in its prevention, Father's plan progresses with relative smoothness as he summons the four independent Human Sacrifices (Ed, Al, Izumi and Van) and also sacrifices Pride's Philosopher's Stone to create and summon a fifth - Roy Mustang. With the five Human Sacrifices gathered around him at the center, Father enacts the final step of his plan as soon as the solar eclipse reaches its peak.
The Nationwide Transmutation Circle activates in Chapter 104, drawing in the souls of every single human being in Amestris to the center. As the Nationwide Human Transmutation occurs, the individual Gate of each Human Sacrifice is forced open, causing them to conflict with each other. This clash releases enough Human Transmutation energy to open the Gate belonging to the Earth itself. As the massive Gate opens under his control, Father expands his body to massive size with the energy of the transmutation and activates the circle in the sky created by the sun's corona around the moon which resonates with the circle on Earth to open the Gate of the Heavens, which Father calls "God". The planetary and celestial gates pull against each other and Father pulls the energy of both Gates into himself. As the dust clears, it becomes apparent that all of the roughly fifty million humans within the country - save for the Five Sacrifices and the few people who happened to be underground, near the center of the circle at the time (May Chang, Greed, Scar, King Bradley, Riza Hawkeye, Jerso, Zampano, Lan Fan and Darius) - have fallen silent and soulless. Father emerges clad in a new, perfect body - resembling young Hohenheim and housing the energy of a country and the power of a god.
Fortunately, a countermeasure put into place ahead of time by Van Hohenheim goes into effect in Chapter 105, activating an array which strips Father's body of all the Amestrian souls he had absorbed and allowing all of the souls to return to their rightful bodies as called back by their spirits. Scar, activating yet another countermeasure, reverses the seal Father had placed on Amestrian alchemy, giving Edward, Alphonse, Izumi and Mustang the ability to fight at their full potential.
- In the anime, the center of the circle is actually much bigger than in the manga; it was able to stretch for at least one block's worth in distance from central command. Although, as Father's pawns/co-conspirators demonstrated, one had to at least be at ground level to be safe.