On May 1st, 1561, King Charles IX of France received a lily of the valley as a lucky charm. He decided to offer a lily of the valley each year to the ladies of the court.
A long-lived pagan custom was to plant an adorned tree on this day to celebrate the return of spring. For the Catholic Church it is the first day of the month of the Virgin.
But by far, all over France, it is the succession of the "Workers’ protest Day" 1889-1947) which officially became Labor Day, la fête du travail, in 1947.