In the US, Memorial Day is celebrated at the end of May and that is when disabled veterans sell poppies (or now, reproduction poppies). Since poppies are actually growing in the fields of Europe at that time, the poppies make perfect sense. I remember the thrill I had the first time I actually saw fields of poppies as I was travelling across France in June during my first trip to Europe. In elementary school we had to memorize and recite In Flander's Fields, so the poppies had meaning to me. BUT, I believe that Memorial Day was originally started as a commemoration of the Civil War, so perhaps the poppy flower symbolism was added after WW I.

To my knowledge, Armistice Day(now called Veterans Day),celebrated in the US on November 11, has no association with poppies.