Milan Expo 2015 Impact

Anything we can say here will necessarily be speculative, since this event hasn't occurred yet, and none of us may be knowledgeable on this subject. Just off the top: I would think that an influx of 20 million visitors to a World Expo in Milan would have an effect on the pricing and availability of hotels in Milan, and would cause more crowding than usual.

[Edit]: You might want to book your Milan lodging as soon as your dates in Milan are firm.

You might also want to directly contact Rick Steves' Europe (RSE) staff who are responsible for booking Milan accommodations for their tours, during the period in question, and see if they have any advice to offer.
They may see your post here, but perhaps you'll want to contact them directly by phone or through other parts of this website.

Eric has helpfully added that the expo will occur over a 6-month period. You'll be there during the last couple of months of the Expo.
It might be reasonable to guess that Milan will get 2 to 3 million additional visitors, in each of the months of September and October when you're there. I seem to recall reading that Milan gets a large number of convention visitors, since it's the economic hub of Italy, and that most of those convention visitors fill Milan's hotels in the months you'll be there (because of the weather they apparently have more conventions in September and October.
If what I recall reading about the timing of Milan's hotel conventional business is correct--all the more reason for you to book your Milan lodging as soon as your nights in Milan are firm.

But I'm guessing, let's see what others think. Roberto and several others here may have better information on this.

Well, it does run for 6 months (May 1st through October 31st), so I would imagine those 20 million people will be spread out some.

As Kent says, nobody knows.

I note that the English language website for the fair now predicts 29 million visitors. Could they just be making up numbers?

29,000,000 divided by 180 days averages 161,111 visitors for each day. That's pretty crowded.

Disneyland averages under 15 million every year, and the fair is only half a year. So the Milan fair hopes to be twice as busy as Disneyland but in only half the time? So on average 4 times as busy? That's ambitious.

We'll see.

Hotel prices are likely to be higher (lots higher) and there will be more crowds, but Milano is big and can handle it. Get your tickets for Leonardo's Last Supper as far in advance as possible if that's on your list.

When I attended the Expo in Lisbon, I booked a hotel from Rick's book by calling about a day ahead, and booked the bus from Seville by showing up at the bus station. When I attended the Expo in Hannover, I'm sure I booked a little further ahead, since it was the first stop of my trip, but not much further, and I did not reserve the train from Amsterdam. No difficulties in either case.

Thanks! We decided to postpone Italy and head for Switzerland and Austria! Happy New Year!

Glad we could help you make that decision.
Happy Travels!