Actually, I can see how Mayday as a religious observance welcoming summer and as a social observance and celebration of work and workers probably arose together.

Simply put, Summer = Work

...If you've ever grown your own crops, or tended your own herds, you know that the start of summer is also the start of the labor intensive season. The work doesn't slow down a jot until the last sheaf is harvested, and the last cow is brought down from the mountain pastures (which is the event that was the initial motivation for the celebration of November 1).

The young men of Padstow,
They might if they would,
For summer is a-coming today,
They might have built a ship
And guilded it with gold
On this merry morning of May!