Millennia before the political people got hold of it, May Day was for lovers.
Equidistant between the Vernal Equinox and the Summer Solstice, arrived that day when human beings participated in the seasonal renewal of life by themselves bursting into bloom—making love.
Details varied. In some places, particularly in the Celtic realm, this day was known as Beltane. Sometimes a woman and man, recognized as particularly sympathetic to or skilled in the magic arts, would, representing the Goddess and God themselves, couple in a ritualized ceremony, either observed or alone, and most often in a freshly seeded field.
Very often, as it says here, “[y]oung couples were encouraged to test their fertility with Beltane trysts, and any babies born from Beltane were believed to be blessed by the Goddess herself.” Pretty magical, such witch children.
Too, “[t]rial unions, called hand-fastings (as the lovers’ clasped hands were bound by ribbon), were also popular at Beltane, committing the couple to each other for one year and a day in preparation for a marital commitment.” Such a ceremony is today popular among some contemporary neo-pagans.
Other places, on this day, there was a sort of relationship “time-out,” when the people of the tribe, in the interest of renewing the earth, could couple indiscriminately, and without consequence.
Of course, “without consequence” is in such things more often a wish, than a reality. In many versions of the Arthurian tale, for instance, Guinevere and Lancelot first acknowledge the inevitability of their attraction on May Day. Fair to say there were some consequences from that one.