But no biggie! The majority of Pagans would point to these dates as Beltane anyway. It makes it easier to organize stuff in advance. No-one is going to sniff at April 30th-May 1st as the time for your Beltane handfasting.
(And if they do, they're being Witchier Than Thou. Laugh in their face and tell them that a High Priestess said to do so.)
However, some of you may wish to keep it as real as possible. Therefore you'll need to skip beyond the Gregorian Calendar to discern when summer begins in your handfasting year.
Many of our Beltane traditions derive from those held in Ireland, Scotland and Newfoundland in Canada, which never did quite die out over the centuries. The rituals in Limerick have especially been a wealth of inspiration.
But if you're looking for a handfasting based on Irish traditions, disdain Beltane entirely. This is the Sabbat which marks the Great Wedding between the Goddess and the God. Do you really want to upstage that?
Plus the Irish thinking is that Beltane weddings are meant to be temporary, Greenwood affairs. If you want your union to last, then May Day might not be the most auspicious date. I've explored some of the legends in the two articles below, which will enable to you to make up your own mind about whether the Irish are onto something here.