Poisson d'Avril!

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In 1509, a French poet referred to a
poisson d’avril (April fool, literally 'April fish'),
a possible reference to the holiday - April 1st
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French cards from the beginning of the
20th century. These 'fish cards' were sent
to friends and loved ones on April 1st.
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Why fish? There are several theories...
Try to read here

French postcard celebrating April Fool's Day, ca. 1907-191.
The post card reads: 'Pretty April Fool's Day Fish,
messenger so discreet / Of he who sends this to
you, guard well the secret'.

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French postcard, celebrating April Fool's Day as it was
done in France - not with pranks but with reminders
featuring fish, which are supposed to be good for one's
memory. The somewhat surprised-looking fish is saying:
'Si vous devinez qui l'envoie,/Votre coeur sera dans la joie'.
This loosely translates as: 'If you guess the sender,
your heart will be joyous'. Postmarked in 1908
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XL embossed April Fools Day die-cut of a Newfoundland Dog
carrying a fish with a babies bonnet, partially mounted on
card with April 1st message attached
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dooo-bi-dooouuuuuuh!