Zandvoort works best on a hot weekday in June or early September — long days, warm sand, and a beach that has not yet filled up with the city's weekend crowds. July and August weekends are reliably packed: a sunny Saturday on the Zandvoort sand means six-deep queues for the train back to Amsterdam.
The other date worth planning around is the Dutch Grand Prix, held in late August or early September on a rotating schedule. Race weekend turns the town orange and books out every bed within thirty kilometres; come a week earlier or later if you want the dunes to yourself. In winter Zandvoort is a different town: most pavilions are dismantled, the boulevard is largely empty, and the beach is the domain of dog walkers and storm watchers — which has its own pleasures if you bring the right coat.