The three-day meeting (30 May-1 June) took three years to prepare.
It will host hundreds of speakers and 7,000 representatives from about 150 countries.
Some 5,400 volunteers will be at hand to help visitors, not to mention local parishes and families who opened their doors to welcome conventioneers.
Contrary to a widespread belief that such large gatherings come to nothing, this meeting "will be felt in Milan, Lombardy and around the world," Card Scola said.
This is because the "normal family, constituted by a man and a woman, tied by bond of fidelity and open to life" is a resource and an asset in contemporary society.