This is unadvertised and may not last very long. Book quickly if something catches your eye. Most third party booking sites have a 24-hour free cancellation policy so you'll have a day to figure out the details.
Travel is available on a wide range of dates, but generally through mid-May and from mid-September onward. For the most part, a 28-day advance purchase and a 7-night minimum stay is required. Some flights may be operated by Star Alliance partner airlines.
As this is pretty much nationwide, check flights from your city, within the travel dates listed above, to see what's available from your area. Unfortunately, most United and American hub cities are excluded.
Generally, prices are in the mid-to-high $400s round-trip from nearly everywhere in the USA to Amsterdam, Paris, Rome, Milan, Venice, Lyon, etc. Even if your favorite airport (small or large) isn't listed, there may be a big price drop. Other cities in these countries are likely lower than normal as well.