When their were red poppy fields in Europe:
https://www.history.com/news/world-war-i-poppy-remembrance-symbol-veterans-day
It has been used more for Memorial Day than Vets day traditionally though nowadays the American Legion sells them for both: https://www.alaforveterans.org/Poppy/
It was inspired by this poem, On Flander's Field:
In Flanders fields the poppies blow
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
We are the Dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved, and were loved, and now we lie
In Flanders fields.
Take up our quarrel with the foe:
To you from failing hands we throw
The torch; be yours to hold it high
If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.
–Lt. Col. John McCrae
A woman, Moina Michael was moved to respond to the poem and wrote:
. . . the blood of heroes never dies
But lends a luster to the red
Of the flower that blooms above the dead
In Flanders' Fields.
She then went on to lobby to make the red poppy the national symbol of sacrifice.
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