Transatlantic airfares are seasonal. DFW-LON in July might cost $1150+ roundtrip; in February maybe $750; in May or Sept. around $950 — for example. Summer is peak season, winter is low season (except around Christmas), and spring/fall are shoulder or off-peak season. Outbound date determines which fare season applies to your itinerary.

So, for example, running a bunch of west coast US city pairs through ITASoftware just now, I see the lowest currently offered is $638 r/t (round-trip) total SEA-CPH outbound by April 1. After April 1, most fares jump $200-$300 or more. Shifting departure by 1 day in this case can offer significant savings.

Seasons vary a bit. I’ve been surprised that so many I’ve checked for 2012 allow departure until April 1. Usually, low season airfares end by the last week of March.

Summer or peak season airfares go up for departures—usually—June 1 or later. Some routes don’t rise until mid-June, and we might still find 1-2 routes that don’t go to their highest summer levels until July 1 and later. So… depart to Europe before June 1 usually helps you stay in the lower shoulder-season fares.

ITA’s month-long search lets you clearly see fare rises between seasons.

Generally, Scandinavia, London, Spain, and Switzerland came up cheapest when I checked just now — for western US to Europe before shoulder season fares rise after ~ April 1 this year.