On the first day of May, a holiday ( Labor Day), the tradition is to give a bouquet of a few Lily of the Valley (" muguet" in French - "meiklokje" in Dutch) to mark spring and to wish good luck to your loved ones, family and friends.
Today, the flowers are everywhere; street vendors offer them at a not so interesting price.. only today, idem in flower shops. They are on tables in restaurants and street cafés, in peoples hands, hats or hair or in men's lapels.
I have been lazy this morning and not been out yet. I hope you forgive my just posting a picture from the net...
"As the story goes, on the first of May 1561, King Charles of France—who was ten at the time—was presented with a fragrant bunch of muguet: the delicate green sprigs capped with tiny white bells that we know as lily of the valley. It was a gesture signifying luck and prosperity, which so touched the king, he continued the tradition by giving the sweet-smelling blossoms to the ladies of his court each year on the same day". http://www.vogue.com/vogue-daily/article/lily-of-the-valley/#1