It would be tempting to call what happened in Chicago between the 20th and the 24th of June in 1926 an "event", but I wont do it, because the term "event" is applied to pretty much anything nowadays.
The 28th International Eucharistic Congress was the largest religious meeting America ever witnessed. 13 cardinals, about 500 bishops, approximately 5000 priests from all corners of the world, 12.000 nuns and one million of Catholics turned Soldiers Field into the heart of the world. Here are some (rather small) images I found:
The Papal Legate, Giovanni Cardinal Bonzano:
Apparently the faithful consumed ten tons of hot dogs during the congress. Trains started to bring the crowds to the ceremonies as early as 4 a.m., running at two-minute intervals. Photos from services held in near-by Mundelein (yes, as in "Cardinal") were published in newspapers already two hours after being taken because of airplane delivery and all kinds of people working like crazy. The choir that sung during opening Mass was made up of 60.000 parochial school children. Fox Films, which distributed a 96 minute movie on the congress, refused to accept any remuneration. Man, that sounds like a healthy Church in a healthy society.