In Holland we call this day 'rokjesdag' or 'bloesjesdag'. That means something like 'mini skirt day' and the second 'blouse day' . The real first warm/sunny day on which you se nice, young and pretty girls with skirts or summer blouses outside. Magical.
Also the special smell of ozone in the early morning, fresh mown grass, ...
And of course the first outside music festival's very important!