Google Flights is showing some reduced air fares starting as early as July & August and running thru the Fall. Change the destination to "Europe" and the length of the trip. Cities with the cheapest fares (at least from my home airport) are Amsterdam, Venice, Madrid, Rome, Paris & London. Delta and American appear to be the airlines offering the best fares. I've seen cheaper flights in the off-season. These prices tend to last for 2-3 days, but may go quicker now, in high season.
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