Muguet, or lily of the valley, is traditionally given in France to wish people good luck and happiness with the arrival of spring.
May Day has been known as La Fête du Muguet since the 16th century when King Charles IX of France declared that anyone could sell flowers - for that day only - without a licence.
That said, the card was sent to me from Finland and on the back are the names of the plant in various other languages:
Convallaria majalis (Latin)
This is for Beth, who hosts Postcard Friendship Friday at The Best Hearts are Crunchy.