The poem was W.H. Auden's. The date marked the moment when Germany invaded Poland, initiating the start of World War II. "September 1, 1939" was originally published in The New Republic on October 18, 1939. You can find the text of the poem here. Plus, you can also read George Orwell's account of what happened that historic day in Poland. I found it, and just had to throw that in.