Looking at buy-up offers for flights in the summer is not particular useful as there is lots of D1 capacity, but not much business travel demand. There are actually several seasonal discount D1 fares available in summer and around US Holidays on many (but not all) US-EU routes. These include ZQN42SDA fares which are valid for outbound 7/10 - 8/17 and returns 7/24 - 9/8. They also have a 14 day advance purchase requirement and 3 day min stay requirement (note that you must meet ALL requirements to qualify for fares). ZQN42SDA fares are demonstrated below on BOS-NAP. Note that the soonest you can qualify for outbound is now 8/7 and beyond departures due to 14 day AP. So in a little over a week, the window for qualifying for these fares will close. Again, these fares are not offered on all route pairs, so you may need to ferret around on Google Flights out of different airports to find them (using map view mode can be helpful in finding valid destinations out of various markets).