Aha a cunning plan a la Len Deighton in Bomber, 31st June being the anniversary of the events described in said book in 1943.
As Deighton pointed out, there was no raid that night that went wrong, there was no village firebombed to destruction using enough firepower to destroy the heart of the Ruhr because a pathfinder got shot down etc, etc.
Because there was no 31st June in 1943 ... or any other year.
Obviously a cunning plan by the hierarchy to get us all to enter a competition that will never be drawn. Well if I don't win, that's what I'll claim.