Thanks for the reply, I probably phrased the question wrong.
From what I could find, Foundation Day is celebrating the construction of the first Amarr church built by Prophet Dano Gheinok.
According to the Amarr Timeline,
EVE Gate opens.
The conformists start migrating to New Eden, spurred on by Dano Gheinok, one of their leaders, Gheinok manages to direct them to the planet of Athra.
Dano Gheinok proclaims himself prophet and sets the foundations of the Amarr theocratic state to come.
The EVE Gate closes as the wormhole collapses.
The same year as EVE closes the last group of conformists arrives on Athra. The conformists all settle on the continent of Amarr, from which they later take their name.
Also according to the Timeline,
EST (Eden Standard Time) and the YC dating system established at Yoiul Conference.
Please correct me if I’m wrong but going by that info, Dano Gheinok proclaims himself as a prophet and sets the foundation of the Amarr theocratic state in 8000 AD, subtract that from when the EST (Eden Standard Time) YC dating system was established in 23236 AD makes that 15,236 years. Add in the current YC date of 123 makes the total amount 15,359 years.
Sooo, for lack of better terminology I think it’s safe to say Amarr Foundation Day YC123 celebrates 15,359 years of Divine Enlightenment.