An American in Provence

In France, it is tradition to offer Muguet, or Lily of the Valley, on the first of May. I had seen a lot of these pots of flowers in the markets, but didn't know the significance until yesterday when Alain's mom gave us a small pot of flowers and explained that it is tradition and that it is supposed to bring happiness.

from http://library.thinkquest.org/C007974/1_1lil.htm
Lily of the valley, May lily, Convallaria majalis
This is one of the most beautiful flowers that appear in wet forests in Europe, Asia, and North America from April till June.Every part of the plant is poisonous because it contains about 20 poisonous glycosides such as convalatoxin, convalarin, and convalamarin, as well as saponins. They cause poisoning characterized by strong headache, nausea and vomiting, slow pulse and excessive urination. The victims are often domesticated animals because the may lily is a widely distributed cultivated plant.

From Wikipedia
The flower is also known as Our Lady's tears since, according to legend, the tears Mary shed at the cross turned to Lilies of the Valley. According to another legend, Lilies of the Valley also sprang from the blood of St. Leonard during his battle with the dragon. Other names include May Lily, May Bells, Lily Constancy, Ladder-to-Heaven, Male Lily and Muguet.

From http://www.placier-muguet.com/uk/histoire.htm
It was not used as a garden plant until the sixteenth century.
Lily-of-the-valley has been associated with French May Day celebrations and regarded as bringing good luck since the renaissance.
But its use specifically as the May Day flower was confirmed at the beginning of the 20th century by two events.
On May Day 1895, Mayol, the cabaret singer, was greeted by his girlfriend Jenny Cook with some lily-of-the-valley and that evening he wore lily-of-the-valley in his buttonhole instead of the more usual Camellia.
At the turn of the century, the great fashion houses gave sprigs of lily-of-the-valley to their customers and apprentices on May Day.
By 1976, lily-of-the-valley was completely associated with May Day and every year tens of millions of sprigs of both wild and cultivated lily-of-the-valley are sold on this day.
The gathering of wild lily-of-the-valley depends on the weather and varies considerably from one year to another.

Alain's parents said that it used to be just kids, poor people, and elderly would gather Muguet and sell them on the streets before May Day, but now this has been almost completely replaced by cultivated Muguet.

Another blog on May Day and muguet.
http://www.frenchgardening.com/postcard.tmpl?SKU=3104967242182928