Red-eyes, preclearance, and Open Skies.

JFK and EWR redeyes

The eastbound transatlantic red-eye departs JFK and EWR between 6:30 and 11:00 PM ET. The earlier you leave, the harder you sleep, because you cross fewer time zones in the dark. Block time JFK to LHR averages 6 hours 50 minutes eastbound, 7 hours 50 minutes westbound; the difference is the jet stream. A 7 PM departure from JFK puts you in LHR by 6:30 AM, which is fine for a hotel that allows early check-in and brutal for a hotel that does not.

Customs and preclearance

Two European countries operate US preclearance: Ireland and the UAE. Dublin and Shannon both clear US-bound traffic before boarding, which means the return into JFK arrives as a domestic flight, no customs queue. Aer Lingus and a handful of US carriers operate the preclearance routes. The desk reads this as a soft-landing premium worth 5 to 10 percent over a non-preclearance routing for travelers connecting at JFK to a domestic flight.

Open Skies and joint business

The US to UK Open Skies treaty plus the EU-US Open Skies agreement let any US or EU carrier operate any transatlantic route. In practice, three joint-business pairs dominate New York: American + British Airways + Iberia + Finnair (oneworld), Delta + Air France + KLM + Virgin Atlantic (SkyTeam, with VS now in the JV), and United + Lufthansa + Swiss + Austrian (Star). Each JV files combined fares, which usually beat the cheapest single-carrier fare for connecting itineraries.

Seasonality

Summer peak runs from late May through early September. The cheap windows are mid-January to mid-March (excluding Presidents' Day week) and the first two weeks of November. Thanksgiving week is expensive in both directions; the week between Christmas and New Year is the single worst week on the index. April and October are the shoulder months the desk prefers for value travel.