I remember three French customs from A Brother for the Orphelines by Natalie Savage Carlson (pictures by Garth Williams, 1959): masks on Mardi Gras, the April fish, and lilies of the valley on the first of May:

"The first of May is Lily-of-the-Valley Day in France. People gather the flowers in the woods and give sprigs to their friends for good luck. Even the president of France gets one, because ever since the days of King Louis IX, the head of the French government has been presented with a lily of the valley on the first of May."

The orphelines look for their lilies of the valley in the Ste. Germaine Woods outside of Paris, where they live in a falling-down house with Madame Flattot and Genevieve to care for them. A Brother for the Orphelines is the third in Carlson's series of books about them: The Happy Orpheline, A Pet for the Orphelines (cats, 12 of them), A Brother for the Orphelines (Josine, the youngest, finds a baby boy in the breadbasket), The Orphelines in the Enchanted Castle, and A Grandmother for the Orphelines. I seem to have liked French orphans, too. Happy May Day!