With many people cutting back on vacation plans this year due to the economy, you should know that there is an almost magical date before which you can save a bundle, and after which you could get soaked on airfares.
Here is part of an advertisement for Alitalia (and hats off to them for being possibly the only airline to advertise complete fares with all taxes and airport fees included). Note the dramatic jump in prices:

Summer rates are nearly $500 higher than spring airfares. What is the magical date when fares turn from rockbottom to ridiculous? The valuable answer is buried in the mouse print:
*MOUSE PRINT:

May 25 this year for some airlines is the last day to fly before higher summer rates begin, particularly for international flights.
On other airlines, for example, fares between NYC and Madrid are nearly $400 more expensive on May 26 than they are on May 25:

Of course, there are dates when the fare comes down again, but generally not to the level they were in the spring.
If you can’t plan your trip to end by May 25, you might be able to still save some money by starting the trip before May 26 and ending it in June. A friend who was flying Continental from New York to Madrid, and then returning from London could have lowered the price of his June trip from $715 to $490 merely by starting it on May 25 — two weeks earlier than planned.
How much could you save by being flexible in your travel dates? Use Kayak.com’s new fare calendar to see at a glance what the lowest airfare is between point A and point B on any day in any month.
Nothing draws attention like a “Buy One, Get One” free offer from an airline. Recently, Spirit Airlines ran a two and a half day sale, from December 22 -24, promising just that.