In France the 1st of April is April fish day. The same idea as April fools day and if you’ve got children young enough, you’re likely to have the drawing of a fish pinned to your back. I think that one of the scenes in the film ‘ The French Connection’ showed the police sorting though a load of smelly fish looking for drugs, the gangster's idea of a poisson d’avril joke.
Henry sent this card to his nephew in 1916, it was written in pencil and must have been sent in an envelope. He was looking forward to seeing his nephew the following Sunday, perhaps he had some leave coming up?