I've put this together to once and for all have a concise timeline available that can be used by my fellow fanfiction writers. I excluded the events that occur in AC3 because my focus lay on Haytham and Shay, but that information can be found by going to the source.
(https://assassinscreed.fandom.com/wiki/Timeline)
For a more compact personal timeline for Shay, all you need to know is that between 1731 and 1757, Shay was born to Irish immigrants. His mother died during childbirth, and due to his father's employment at the merchant marine, he was raised by his aunt. 8 years later, he meets Liam O'Brien and joins his father on occasional travels. His father's life is eventually claimed by a storm, pushing Shay into a deep depression, to which Liam offers him to join the Brotherhood.
Achilles Davenport, a former student of the Mentor Ah Tabai in the Caribbean, established the Colonial Brotherhood of Assassins in the Thirteen Colonies and assumed the title of Mentor.
June 11: Juan Vedomir, a Templar assigned to decode Edward Kenway's journal, was killed by Haytham Kenway for working with the Assassins. Haytham took the journal with him, and later brought it to Reginald Birch in the Templar headquarters in Prague.
September: During the retreat from the Siege of Bergen op Zoom, Edward Braddock ordered the execution of a family asking for passage on his boat. Braddock's relationship with Haytham Kenway deteriorated as the latter feared Braddock was turning away from the Templar Order.
The Assassin Vendredi discovered and disturbed the Piece of Eden in the Isu Temple near Port-au-Prince, on the orders of the Mentor François Mackandal, triggering an earthquake that destroyed the city. Vendredi was killed by the Master Templar Lawrence Washington during his escape.
Lawrence Washington stole a Precursor box and the Voynich manuscript from the camp of François Mackandal in Saint-Domingue.
January: The Assassins Shay Cormac, Liam O'Brien, and Louis-Joseph Gaultier, Chevalier de la Vérendrye rescued a group of allied smugglers from the Royal Navy. Shay stole a Royal Navy sloop, the Morrigan, and made it his flagship.
March: The Assassin Adéwalé met the Mentor Achilles Davenport of the Colonial Brotherhood at the Davenport Homestead to inform him that their Precursor box and the Voynich manuscript had been shipped to North America. Adéwalé then returned to Saint-Domingue with supplies to help victims of the earthquake.
April: After meeting with their associate Le Chasseur, the Colonial Assassins discovered that the Voynich manuscript was in the possession of Lawrence Washington.
July: Lawrence Washington organized a meeting with his fellow Templars at his estate in Mount Vernon to entrust them the Precursor box and Voynich manuscript. After the meeting, Washington was killed by the Assassin Shay Cormac.
June 25: Haytham Kenway abducted Lucio Albertine to help his mother, the captive Monica, a codebreaker and Assassin associate, decode Edward Kenway's journal for the Templars. Haytham stole the Hidden Blade of Miko, Lucio's Assassin bodyguard, while engaged in combat with him.
April 18: Haytham Kenway assassinated Miko during a performance of The Beggar's Opera at the Theatre Royal in London, and retrieved the Grand Temple's key from him.
April 20: Haytham Kenway boarded the Providence to start his 72-day long journey to the New World to find the Grand Temple. During the journey, Haytham killed the Assassin sailor Louis Mills and managed to evade capture by the Aquila, luring it to its destruction in a storm.
May: The Templar Samuel Smith was killed by the Assassins Shay Cormac in Terra Nova. Shay retrieved the Precursor box from Smith
The Templar James Wardrop was killed by the Assassin Shay Cormac during the Albany Congress in Albany, New York. Shay retrieved the Voynich manuscript from Wardrop.
Benjamin Franklin, with the use use of electricity and help of the Assassins Shay Cormac and Hope Jensen, made the Precursor box functional. Reading the Voynich manuscript, the box displayed a holographic map of the globe, highlighting Port-au-Prince, Lisbon and the Arctic.
July 8: Haytham Kenway arrived in Boston, Massachusetts, and formally took up the position of Grand Master of the Colonial Templars, recruiting Charles Lee, William Johnson, and Thomas Hickey.
July 10: Haytham Kenway and Charles Lee rescued the surgeon Benjamin Church from capture by the slaver Silas Thatcher, and recruited him into the Colonial Templars.
July 13: Haytham Kenway recruited the soldier John Pitcairn into the Colonial Templars, after attacking and threatening his former comrade Edward Braddock in order to get him to release Pitcairn from service.
July 14: The Colonial Templars besieged the fortress of Southgate in Boston and killed the slaver Silas Thatcher. They then freed all of Thatcher's slaves, including a Kanien'kehá:ka woman named Kaniehtí:io.
November 15: Haytham Kenway formed an alliance with Kaniehtí:io, who claimed she could lead him to the Grand Temple, by planning a joint attack on Edward Braddock.
July 9: General Edward Braddock was fatally wounded by Haytham Kenway during the Braddock Expedition, with aid from Haytham's fellow Templars, Kaniehtí:io, the Abenaki, Shawnee, and Lenape, for his betrayal of the Templar Order's principles. Braddock died four days later.
July 10: Kaniehtí:io took Haytham Kenway to the entrance of the Grand Temple, which the latter tried to access using his key, but was unsuccessful.
July 13: Charles Lee was inducted into the Templar Order by Grand Master Haytham Kenway.
August 4: Charles Lee met with Haytham Kenway to confirm Edward Braddock's death. Overhearing their discussion, Kaniehtí:io realized that Haytham had lied about making sure Braddock was dead and, believing Haytham had used her, cut ties with him.
November 1: The Assassin Shay Cormac disturbed the Piece of Eden in the Temple beneath the Carmo Convent in Lisbon, causing an earthquake that destroyed the city and killed thousands of citizens. Shay managed to outrun the earthquake but was left severely traumatized.
Winter: The Assassin Shay Cormac stole the Voynich manuscript from the Davenport Homestead, believing that Achilles Davenport knew about the Seismic Temples and was indifferent to putting innocent lives at risk. After discovering the theft, Achilles ordered his Assassins to capture Shay and they cornered him atop a cliff, where Shay was presumed dead after falling into the ocean.
Shay Cormac was nursed back to health by Barry and Cassidy Finnegan at the request of Colonel George Monro, a Templar. On the insistence of Monro, Shay started financing the renovations of New York.
Shay Cormac rescued Christopher Gist from hanging by the Assassin-affiliated gangs in New York, from whom he reclaimed the Morrigan, and seized Fort Arsenal as his new home.
Le Chasseur, a smuggler and Assassin ally, was killed during a Templar attack on Fort La Croix by Shay Cormac, to prevent him from warning the Brotherhood of Shay's survival.
Shay Cormac received a prototype grenade launcher from Benjamin Franklin and used it to destroy a factory in New York which was being used by the Assassin-affiliated gangs to manufacture poisonous gases.
Shay Cormac infiltrated Fort de Sable, where he found and retrieved the armor worn by the Templar James Gunn during his expedition to North America in 1398.
Shay Cormac retrieved a set of 11th-century Templar armor from a shipwreck in the North Atlantic, after finding all the keys to the chamber where the armor had been locked away.
August 10: George Monro and his men escaped an Abenaki ambush led by the Assassin Kesegowaase with the aid of Shay Cormac, after being released following their surrender at the Siege of Fort William Henry.
September 17: Jennifer Scott was rescued by her brother Haytham Kenway and Jim Holden from servitude at the palace of Governor As'ad Pasha al-Azm of Damascus. However, they were detected by the guards and Holden was captured while buying Haytham and Jennifer time to escape.
September 21: Haytham Kenway rescued Jim Holden from the Abou Gerbe monastery on Mount Ghebel Eter in Egypt, where Holden had been castrated and buried in sand. In retaliation, Haytham set fire to the monastery and killed all the priests present.
Grand Master Reginald Birch was killed in his chateau near Troyes, France by Jennifer Scott, with the help of her brother Haytham Kenway, for having been responsible for their father's death and selling her into slavery.
Haytham Kenway freed Reginald Birch's prisoners Lucio and Monica Albertine, but was impaled with a sword by a vengeful Lucio. He was nursed back to health over the course of several months by Jennifer Scott and Jim Holden.
November 3: The Assassin Kesegowaase was killed by Shay Cormac in Albany. The Templar George Monro was killed in a fire lit by the Assassin Liam O'Brien, who stole the Voynich manuscript from Monro.
January 20: François Mackandal, Mentor of the Assassin Brotherhood in Saint-Domingue, was publicly executed after attempting to poison the white inhabitants of the colony. His pupils Agaté and Baptiste fled to Louisiana.
January 28: Once Haytham Kenway had made a full recovery, Jim Holden committed suicide, as he was no longer able to cope with the injuries and trauma inflicted upon him at Abou Gerbe.
Haytham Kenway returned to the Thirteen Colonies, where he inducted Shay Cormac into the Templar Order.
June 8–July 26: The Siege of Louisbourg was fought between the Royal and French Navies. The Templars, using James Cook's HMS Pembroke, thwarted an attack by the Experto Crede captained by the Assassin Adéwalé and secured a British victory.
The Assassin Adéwalé was killed by the Templar Shay Cormac at Vieille Carrière, with the assistance of Grand Master Haytham Kenway, after beaching his ship, the Experto Crede.
October: The Assassin Hope Jensen was killed by the Templar Shay Cormac following an altercation inside her mansion in New York. The Assassin Liam O'Brien returned to Achilles Davenport with the location of another Isu temple retrieved from their Precursor box.
The Assassin Louis-Joseph Gaultier, Chevalier de la Vérendrye was killed by the Templar Shay Cormac in the North Atlantic during an expedition to distract the Templars from their search for the Precursor temple in the Arctic.
March: A Seismic Temple in the North Atlantic created an earthquake after its Piece of Eden was disturbed during a confrontation between the Assassins Achilles Davenport and Liam O'Brien and the Templars Haytham Kenway and Shay Cormac. Liam was killed by Shay, and Achilles was shot in the leg by Haytham, who decided to spare the Mentor at Shay's request, so that they could warn the other Assassins of the dangers of the Seismic Temples.
Colonial Templars led by Charles Lee assaulted Ratonhnhaké:ton, Haytham Kenway's illegitimate son with Kaniehtí:io, during their search for the Grand Temple in the Mohawk Valley. After being unable to locate the site, the Templars eventually abandoned their search.
The Kanien'kehá:ka village of Kanatahséton was set aflame by George Washington of the British Army, killing many villagers including Kaniehtí:io. Unable to save his mother, Ratonhnhaké:ton blamed Charles Lee and the Templars for the attack and vowed revenge.
