It is probably a relic of some old sacrificatory festival to Maia, who has given her name to this month, -- the custom still remaining after its significance is gone.    From Wordnik.com.  [The Atlantic Monthly, Volume 05, No. 31, May, 1860] Reference

It is, however, believed that it derives its origin from a heathen sacrificatory festival; and there is ground for the acceptation that children were sacrificed alive at this very feast, -- and this, in fact, in order to expel or reconcile the evil spirits, of whom the people believed, that, partly flying, partly riding, they commenced their passages over fields and woods at the beginning of spring, and which are to this day called enchanters, witches, nymphs, and so forth.    From Wordnik.com.  [The Atlantic Monthly, Volume 05, No. 31, May, 1860] Reference