The main historical story of the first Assassin's Creed game takes place in 1191.
The year 1191 places Altaïr Ibn-La'Ahad's missions during the Third Crusade, a conflict involving Christian European forces and Muslim powers in the eastern Mediterranean. The game presents a fictionalized version of this period, with Altaïr moving between Masyaf, Damascus, Acre, and Jerusalem while investigating targets connected to the Templars.
The date is important because it separates the original game from later historical settings in the series. Assassin's Creed II moves to Renaissance Italy, while Assassin's Creed Origins reaches back to first-century BCE Egypt. The original game's medieval architecture, political tension, and travel between Levantine cities all reflect its Crusades-era setting.
Some players confuse 1191 with 1099, the year of the First Crusade's capture of Jerusalem, or with the modern-day scenes involving Desmond Miles. Those are different chronological contexts. The historical simulation that forms the main adventure is specifically set in 1191.