It's not about what happens on May 1, it's about what it means. It's akin to like June 1 being the start of hurricane season...what happens on June 1 in terms of tropics? Nothing necessarily but it designates the start of the month where activity can start increasing. Outside of the Earth's tilt of the axis, what exactly happens on the first day of spring, summer, fall, winter...nothing really. It just marks a period in the transition.