Best Time to Purchase Airfare

There is no answer to that question. Buy it when the ticket is at a price you can afford. Go on farecompare to see what the historical price has been but that may be only a fair indicator going forward.

You are traveling during shoulder season which is a lower fare in general. I would check now, track your findings and compare with a consolidator like cheaptickets. Remember certain days of the week are cheaper.
Also, I have seen some specials like Lufthansa had a one day sale yesterday. Your arrival and departure airports in Europe can make a big difference in your fare. Frankfurt, Dusseldorf and Dublin are generally lower fares from rest of europe. When you find a fare that looks reasonable to you buy. If you are stuck call an airlines they can give you some guidance.

Yeah, the answer really is when you see a good price, jump on it. But...generally...somewhere around 12 weeks before departure is a good balance between declining prices and good availability. Be aware of price points around the time you are going, there is usually a jump some where around the middle of April, another about the beginning of May, then the middle of May, etc... As someone mentioned, Day of the week matters, I usually leave on Wednesday or Thursday and return on a Monday. Also mentioned was that there are certain cities that are cheaper to fly into, but really only if it works for you. If you have to pay to get by train where you want to go, savings may evaporate. How do you know a good price? Historical prices on fare compare, maybe, but by tracking yourself you have more current info.

I use www.kayak.com for hotels, airfare, cruises and packages.

Kayak finds the cheapest (to most expensive) deals taken from numerous websites.

I have personally never purchased from kayak I just like to use it as a guideline.

Sometimes you can find super cheap last minute deals but when I'm traveling overseas I'd rather make sure I have my ticket ahead of time instead of risking it :) I'm not spontaneous when it comes to plane tickets :)

I use sidestep.com which might be the same as kayak now. It has a feature to send you a daily fare alert. I set that up and watch for awhile to see when I think it's the best time to purchase.

I always set up an alert for multiple airports on both ends and at least a range of several days. The more flexibility you have, the better your chance to grab a great fare.

With all the cancelations these days it may be better to buy your ticket no more than six to eight weeks ahead of your trip to reduce the chance that your flight is canceled and you have to rearrange your schedule.

Even if you wait until then, there's no guarantee the airline won't cancel your flight. It's seems to be getting more common to get an email or phone call that your flight is canceled and you need to book a different flight (for the same fare). The worst case scenario is they cancel your flight, refund your ticket and you have to start looking for another flight at current fares. I suspect hotels are getting a lot of late cancelations because of flight changes.

I don't think there is a best time either. I just purchased (on line) tickets for my sister's and my trip to the UK in Sept 2009 on British Air. I paid $200 more than I would have last week, but, while the fare from SEA to LHR was great(ish), the fares home were not yet published. By the time they were, the fare for my preferred departure date had gone up. So I'm going to live with it and not check fares again!! Or should I? Now I have a headache.

We bought our tickets for BA flight to London from Seattle in April for our trip in September. I didn't know at the time that they don't let you choose your seats until 24hr before flight departure, but we were still able to get good seats on the plane anyway.