These discount summer roundtrip D1 fares are actually available right now with fare basis code ZNX82SDA for mid-week travel (Tue-Wed outbound and Tue-Thu returns) and ZNW82SDA for weekend travel which will cost a few hundred more in each direction of travel. They are valid for travel from July 9th to August 31st and require a Saturday night stay and fares must be booked by May 15th. The Saturday night stay requirement is from point of arrival (so must depart Friday or earlier in week from most US origins to meet this if stay is less than a week).

Additionally, there are discounted D1 fares around significant US holidays. The currently published ones are valid around Memorial Day, 4th of July, Labor Day, Halloween, and Thanksgiving. The valid dates for travel are covered in the seasonality section of the fare rules. These fares have fare basis code ZJN72SDD. These also have a Saturday night stay requirement and also must be booked by May 15th.

All this being said, OP did mention JFK (which is a different animal) and tends to have discount D1 fares throughout the year to many business centric destinations. For example, the current cheapest ones on JFK-LHR just have a 50 day advance purchase requirement and Saturday night stay requirement. Some routes have a 60 day advance purchase requirement for cheapest D1 fares. So in the specific case of JFK, one should mainly focus on booking at least 60 days in advance and including a Saturday night stay in trip.

Fare rules are found at the following link on delta.com when pricing out fares.


Fare rules for summer travel D1 fares.







Here is an example of holiday seasonal roundtrip D1 fares.







Cheapest roundtrip D1 fares on JFK-LON