Twenty-one years old, carrying artillery heavier than any yet cast and ships hauled overland into the Golden Horn, Mehmed II pressed the siege of Constantinople for fifty-three days. Before dawn on Tuesday 20 Jumādā al-Ūlā 857, the third assault carried the land walls near the gate of St Romanus; the last emperor died in the fighting, and by midday the sultan rode to Hagia Sophia and prayed there.
The city eleven centuries of armies had failed against — including the first sieges of the Umayyads, where Abū Ayyūb al-Anṣārī's grave lay outside the walls — became the Ottoman capital. Mehmed took the title "Abu'l-Fatḥ", father of conquest, and the tradition connected the day to the Prophet's ﷺ praise of the army that would take the city.
Both Greek and Ottoman sources fix the Tuesday exactly.