May 1 is “May Day” in France. “Muguet” (lily of the valley) has been symbolic of spring and renewal since at least the middle ages. Lily of the valley is also considered to bring luck. Traditionally people in France give bouquets of lilies of the valley to people they love on May 1st for good luck and to celebrate the arrival of spring. This tradition is said to have originated with Charles IX in 1561, although celebrations marking May 1st and the giving of spring flowers of some sort on this day existed long before Charles IX.
You can watch a video from TF1 of French people buying lilies of the valley to give to their friends and family here: Achats de Muguet