Roadside stands sprung up and greengrocers had bunches of these for sale today, May 1, Labor Day in Europe. I asked a few people why this particular flower, and no one could say definitively. And so to my trusty source, Wikipedia:
King Charles IX of France was presented with lily of the valley flowers on May 1, 1561. He liked the gift and decided to present lily of the valley flowers to the ladies of his court each year on May 1. Around 1900, men started to present a bouquet of lily of the valley flowers to women to express their affection. The flowers are a more general token of appreciation between close friends and family members these days.
Gotta love a country that keeps a tradition going from the 1500’s! We’re not even that old in the US!