Traveling time is essential to consider when traveling abroad. I would assume most people would not want to pay more for a vacation and deal with crowds. Well- that is what you get if you go to Europe during the summer. Yes, I have traveled to Europe for a couple of times during the summer and it was fun. But, now as I get older it has lost it's appeal.
Best time to travel to Europe May or October. Why? The weather is not so bad and there are virtually no crowds and everything is less expensive.
I remember waiting in line for the St Peters or the Vatican for hours during a Summer and discussing with my friend who went to the Eiffel Tower and said the line to get to the 1st tier was hours long. I went in May and October to Rome and Paris and no lines. I walked right up to the Eiffel Tower no wait. Yes, yes it was colder, but imagine what you can do with your time. I can sleep in deal with jet lag and explore experience new things because I am not sitting in lines.
Great resources in my links -- you can look at the big websites about timing your trip abroad with Frommers, Lonely Planet, and Fodors.
Other things to consider is what events or holidays are going on that might interfere or enhance your trip. I have a friend who is a Formula 1 fanatic. We try to arrange trips in Europe to coincide with the cities it is in. Some holidays can be fantastic to see. My wife and I planned out seeing a Saints procession in Sevilla off - season - what a find. It was a parade of part of the town carrying massive statues to honor the patron saints. Quite something to see. Since it was off-season, we stood right up front and saw everything. If you go for the major procession, it is a mess of people and very difficult to see. Unless you stand out all morning and stake out your spot.
Other fond memories include spending time at the hotel pool, relaxing at cafe, playing at the kids at a park and then entering the Riks Museum in Netherlands. Reason being off-season. Again it was cold, but my friend that traveled during August spent most of the day waiting in line for Anne Franke's house while we spent all day relaxing and just walked in.