Origin of the symbol
The association of the poppy with the Great War is rooted in its first year, during the second battle of Ypres. Lieutenant Alexis Helmer, a Canadian artillery officer, was killed by an incoming shell in May, 1915. His friend, the unit's doctor, Maj. John McCrae, was asked to preside over the graveside service for Helmer. Soon after this McCrae composed a short three stanza poem, In Flanders Fields, the first stanza of which is:
In Flanders fields the poppies blow1
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.
1 - some versions print grow
The poem became one of the most quoted poems from the war.
The European field poppy, Papaver rhoeas
As the war ended in 1918, American professor Moina Michael, inspired by the poem, began wearing a red silk poppy as a remembrance of those who served in the war. She promoted this at various events, and at one a Frenchwoman, Anna Guérin, took up the idea and began selling artificial poppies as remembrance symbols. In 1921, she began offering them in London, and Field Marshal Douglas Haig began wearing them. From there, the tradition spread to Canada, Australia and New Zealand.
The choice of November 11 as the most common day for wearing the poppies is because that was the day the Armistice of 11 November 1918 took effect.