If you are flying to Europe, you'll need it when you get on the plane and when you get to passport control in the first airport where you land. If you travel to other countries within the EU, you won't need a passport again.
I don't know how they do it if you are cruising from the US to Europe.
I asked a similar question about whether cruises in Europe (which we'll do in Sept) are like the one we did in the Caribbean, ie we showed our passport upon boarding the ship and never had to do it again, even tho we were in 3 different countries. The answer was that it will be the same in Europe (I was hoping for some new stamps!) I'm still not sure how this works! Clue us in if you find out.
JM