This is a short story written by J.D. Salinger, of The Catcher in the Rye fame. The story revolves around a man who has recently returned from active combat and his fiancée. They take a trip to the beach, where he encounters a sweet, innocent little girl who is maybe four years old. He warns this child against the dangers of "the bananafish." All bananafish, it seems, look identical, and have a similar taste for bananas. He says that the bananafish often stuff so many bananas into their mouths that they can not get in and out of the holes in the sand where they live. The protagonist realizes that it is harmful to the fish, and presumes that they often die.
The symbolism behind this is that once soldiers kill people, it is hard for them to go back to their daily lives when they return from the war. The identical appearance of the fish is representative of the army fatigues that all soldiers wear. The girl, not understanding this, says that she saw a bananafish swim by with six bananas in his mouth. The soldier returns to his hotel room, sees his fiancée asleep on the bed. He pulls out a pistol, puts the barrel against his temple, takes one last look at his fiancée, and fires. The title suggests that it was a perfect day for the veteran to kill himself.
This is essentially an anti-war story outlining the psychological damage done to verterans, as well as their families.