https://www.flyplay.com/world-tourism-day?Dates and rules in the link.
I flew them two weeks ago and they were fantastic. Obviously after paying for everything it came out around double the original cost, but it was still less than half of any other airline.
Keeps on mind that they arrive in Iceland very early, it might make sense to take a hotel room upon arrival for a few hours. And even with that it is still crazy cheap.