Just to be clear, the holiday seasonal award levels are not really one-off "sale" awards and the patterns are fairly predictable by looking at a calendar for holidays. These are simply periods when business travel demand is low and they attempt to sell otherwise empty J seats to the EU to more price sensitive leisure flyers. Google Flights can also be quite useful for showing them as well using calendar view since awards coorelate well with revenue fares. The validity dates can be found in the Seasonality section of the fare rules (posted above). At this point, they haven't yet filed holiday D1 fares/awards beyond Easter for 2025, but if previous patterns hold, you can expect to see them also around Memorial Day, 4th of July, Labor Day, Halloween, Thanksgiving, and Christmas, in addtition to President's Day and Easter.

In addition to DTW-CDG for 221K roundtrip, you can also find 221K roundtrip D1 awards on ATL-MUC. While not quite as low as what you can find at times out of competitive hubs like BOS and JFK, they aren't as bad as you would normally expect out of hostage hubs.