I don't know which "Green Left" you are referring to, but this one, for example, makes no attempt to associate the modern working class celebration of May Day with Beltane.Beltane was not "supplanted" by May Day any more than Hannukah was supplanted by Christmas. Both have been celebrated or observed contemporaneously by different people in different places at different times and for different purposes.
May Day commemorates the advent of the 8-hour working day, effective May 1, 1886 in the USA. It was a major victory for the industrial working class. That struggle had nothing to do with pre-industrial pagan spring celebrations, nor did the legislators who selected May 1 as the starting date have such celebrations in mind.
To suggest that the international working people's holday known as May Day had "pagan roots" is utter historical revisionism, free from any taint of supporting evidence. It also denies the true history and importance of the struggle for the 8-hour day.
Thanks to the way history is taught in our schools, workers are sadly uninformed about their own history of class struggle. Their traditions and accomplishments are purposely buried and hidden from them. They are not helped by those who spread falsehoods and myths about labour history.
If you want to recover the tradition of earth celebrations, there is Earth Day on April 22, a week before May Day. I think this is an entirely worthwhile endeavour.
But the tradition of labour struggle in North America is something very different. It is not just a mere historical curiosity, but a very relevant aspect of industrial working class culture that deserves to be kept alive.
May Day ought to be recognized on its own as something quite separate and apart from any other traditions, including those of ancient mystical and agrarian myths.