In the current economic climate, I'd leave it as long as possible - maybe late summer (early September when all the summer peak travel in the States is over) and then start looking for a bargain. We're going to Phoenix in April for a wedding and because we were tied to dates, booked in December and flights cost €1,200 return - same journey can now be done (with different stops) for half the price. Never can tell - but my advice as a twice-yearly traveller to the States, usually on the basis of flying at least cost with flexible dates - is, leave it as long as possible, unless of course, there's an almighty sale in Transatlantic prices. Have you checked the prices from the UK/Amsterdam - more traffic sometimes brings the price down.