The London Lounge

Does anyone know when air fares to Europe are cheapest, I mean the exact dates?

Maybe October, November?

The best time for rock-bottom fares is January/February, but any time from late autumn to early spring (except Christmas/New Year) should be fairly reasonable. The cheapest tickets are usually on the routes with the most capacity and competition -- JFK/LHR is usually the best deal, with other cities marginally more expensive.

Subscribe to a couple of your favorite Transatlantic carriers' email lists for specials and the like. (Mentally) commit to a week or long holiday weekend in January or February. When the
email specials arrive, book and go. You shouldn't have trouble getting hotel rooms in most
cities with short notice, since it's out of season. At least that has been my experience with Rome and London (though I have used hotel points in London too). I suppose you could always book the hotel ahead of time and cancel if needed.

Of course, getting a time with your tailor and shirtmaker on short notice may be a different story.