Remembrance Day (also known as Poppy Day Or Armistice Day) is a memorial day observed in the Commonwealth countries since the end of World War I to remember and pay respects to the many members of their armed forces who had died in the line of duty. Remembrance Day is observed on 11 November to recall the official end of the World War I on that date in 1918, hostilities officially ended at the 11th hour of the 11th day of the 11th month.
The red remembrance poppy has become a tradition of Remembrance Day due to the poem 'In Flanders Fields'. These poppies grew across some of the worst battlefields of Flanders in the war, their red color an appropriate symbol for the blood spilled in the war.
(sourced from Wikipedia)