It's a Dutch holiday celebrating the birthday of the king on April 27th! Our national colour is orange which is everywhere that day, because the royal house is "Oranje Nassau" - oranje = orange. There's festivals, music, activities, fun all round - but most importantly: Vrijmarkt. Aka free market aka mega flee market. It's THE day for kids, adults and anyone in general to claim a spot in their city or towm, set down a blanket and sell *anything*. Including of course, vintage toys, such as ponies!
Dutch cities don't compare in size to American ones, but it's big enough to occupy multiple main streets of most cities, people getting up at the crack of dawn to claim a spot (or even marking it with chalk or tape DAYS before) and sell.
So yes, the ultimate pony hunting ground! I often scour for Pokémon cards and secondhand books as well. Anything goes, really.
Great to earn some pocket money as a kid too!