When is the best time to book an International flight for the Fall?

I would start looking now because as the seats start filling up, the ticket prices start going up. You definitely want to have bought your ticket at least 8 weeks before your departure date.

Fares to Europe usually drop at mid-September. A quick glance at the lowest daily fares for a 14 night stay (Chicago to London) has $900 to $950 until Labor Day, $850 to $900 the two weeks following that, and $782 to $857 for the last two weeks.

It is doubtful the fares will drop from now for that time period. They are far more likely to increase as the date gets closer. Airlines habitually revel in sticking it to people who don't plan ahead.

The only retribution passengers have is identifying flights which do not fill and offering like $50 within 24 hours of flying. Basic economics is that this is better than taking off with that seat empty since the incremental cost of another passenger is close to zero (minuscule increase in fuel consumption and the price of complimentary peanuts).

Another reason for airline fares to drop is some crisis. If the uprisings in Greece continue and are covered well enough in the U.S., it could cause fares to drop to Athens (perhaps also eastern Italy) if enough Chicken LIttles decide not to travel. In case it is not obvious, a flight landing in Athens airport is not affected by protests 15 miles away in central Athens, so flying to Athens and connecting to a flight to, say, Vienna, would leverage popular misperception to your benefit.

Most airlines release their first batches of tickets about ten months prior to departure and as these sell out they release more at increasingly higher prices. Get online and check out the comparison sites like Expedia.com and others. When you find a deal that suits you go to the airline's website and check out the price there. It could be cheaper If you see a good price, buy it. It may not be there in a few hours.

Airlines always increase their prices during public and school holidays so avoid those times as much as possible.

Book air ticket in advance, this is the best way to find the cheap air ticket.