Dating uses years Before the Genocide (BG) and After the Genocide (AG). Year 0 is the year Sozin uses the comet to begin the Hundred Year War with an assault on the Air Nomads. This campaign starts in 12 BG and looks toward that date as a threat, not a script.
82 BG. Avatar Kyoshi dies at the age of 230. Avatar Roku is born in the Fire Nation, along with his twin brother Yasu (Yasu later dies at twelve, swept out to sea in a storm). Sozin is born the same day as Roku, son of Fire Lord Taiso and Fire Lady Hazei. Sozin also has a younger sister, Zeisan, a nonbender.
66 BG. At sixteen, Roku is identified as the Avatar by the Fire Sages and leaves to master the other elements. Sozin gives him the Crown Prince headpiece as a sign of friendship. Roku trains in air under Sister Disha at the Southern Air Temple and slowly befriends a young monk named Gyatso. The novels The Reckoning of Roku and The Awakening of Roku cover much of this youth: Lambak Island, Ta Min's early diplomacy, Sozin's ruthlessness, and Roku learning what it means to be a spirit of no nation.
About 63 BG. Roku begins waterbending training in the North. Taqukaq becomes his master. Gyatso travels with him for a time. Fire Lord Taiso declines; Sozin gathers power as prince.
58 BG. Taiso dies. Sozin becomes Fire Lord. In the early years of his reign the throne wins the last real conflict with the noble clans (General Oraso Eiko is named in era sources). Sozin uplifts some of the poor, begins a coal-powered fleet, and also starts the tradition of dragon hunting to bind the nobility. Construction of the Fire & Air Center of Learning, requested in Roku's youth, becomes a political flashpoint with the Guiding Wind.
55–54 BG. After mastering earth with a teacher named Sud, Roku spends months at the Crescent Island Fire Temple under Sage Kaja trying to master the Avatar State, nearly destroys the temple, and triggers a volcano. He becomes more cautious afterward. In 54 BG he returns to the Fire Nation after twelve years away and marries Ta Min. Sozin is best man, then takes Roku aside and proposes that the two of them spread Fire Nation prosperity as an empire. Roku refuses: the four nations are meant to be four. Sozin asks him to consider it. Roku says he never wants to hear of it again.
In these same years, era sources describe a Northern Passage crisis between the Northern Water Tribe and the Earth Kingdom state of Chenbao; sabotage and rivalry between Earth King Jialun and Queen Guo Xun of Omashu; floods in the southern Earth Kingdom; and Air Nomads spread thin as mediators. Jialun, earlier, had used the Dai Li to purge the Earth Sages in the Night of Silenced Sages. Those events are recent history by 12 BG, not necessarily still staffed by the same monarchs.
37 BG. Sozin invades the western Earth Kingdom and plants colonies, including Yu Dao. Roku discovers this, confronts Sozin in the palace, is attacked from behind, defeats him, levels the throne room in the Avatar State, and spares him with a final warning. They do not speak again for twenty-five years. The first colonies remain and grow.
12 BG — now. Roku is seventy. Sozin is seventy. The island erupts. Sozin leaves Roku to die. Fang dies with him. The next Avatar is born. This is the first hour of your campaign.
50 BG, for reference: Guru Pathik is born. In 12 BG he is thirty-eight, a friend of Gyatso and a spiritual brother to the Air Nomads, not yet the ancient hermit of a later century.
0 AG, twelve years ahead if not stopped: Sozin's Comet, a once-a-century event last seen in 100 BG. Sozin intends to use it. Azulon, Iroh, and Ozai are not yet born at campaign start. Azulon will be born shortly after the war begins, very late in Sozin's life.