Don’t know about FB and could not read any more on the link other than what I copied, so I don’t know who/what suggested it. I just searched the French, remembered it was some poem I read long ago (a depressing one of course) and realized what you meant!
https://en.wikipedia.org/wiki/Chanson_d%27automne
“Chanson d’automne” (”Autumn Song”) is a poem by Paul Verlaine, one of the best known in the French language. It is included in Verlaine’s first collection, Poèmes saturniens, published in 1866 (see 1866 in poetry). The poem forms part of the “Paysages tristes” (”Sad landscapes”) section of the collection.[1]
In World War II lines from the poem were used to send messages from Special Operations Executive (SOE) to the French Resistance about the timing of the forthcoming Invasion of Normandy.
More details here:
https://www.vox.com/2014/6/6/5785954/how-paul-verlaine-helped-the-allies-pull-off-d-day
Yeah, so I gave you the second line. When they heard the first line they were sipposed to prepare, when they heard the second line it meant the invasion was on.