When doesn’t Sunday fall on a Sunday? When it’s Sun Day, May 3 — a special day set aside every year to appreciate solar energy. As nations around the world look for ways to achieve net-zero emissions, one of the greatest sources of clean energy is beaming down from the blue sky above — on clear days, of course.When doesn’t Sunday fall on a Sunday? When it’s Sun Day, May 3 — a special day set aside every year to appreciate solar energy. As nations around the world look for ways to achieve net-zero emissions, one of the greatest sources of clean energy is beaming down from the blue sky above — on clear days, of course.