The rarities season

Summer migration in Holland starts mid-April. At least that's when the numbers of migrating birds start increasing seriously. The peak of summer migration usually is between the last week of April and the second week of May.
There are several good migration watchpoints, of which Breskens is the most famous one. At Breskens birds migrating north are forced to make a decision: either cross a large body of water (the Westerschelde) or keep following the coast line and turning more to the east.
Most birds choose the latter and on a good migration day with Easterly or South-easterly wind force 3 or 4, hundreds of thousands(!) of migrating birds can be seen here.
Very impressive if you're into watching migration or if you're hoping to catch a glimpse of a rarity for the Dutch list flying by, but other than that not very interesting for the average visiting birder.

The interesting part about the migration apart from experiencing the change of the seasons, is the chance that a rarity may pop up. Anything, anywhere.

Dutch listers live for the next rarity, hoping they can add yet another tick to their already impressive lists, but to the unwary visitor the rarities season can also have some unexpected bonuses for their birding trip to Holland.
This bonus can vary from "oh, nice" for the odd Melodious Warbler, to "bloody hell" for a bird you never imagined possible like a Bulwer's Petrel or a Red-headed Bunting.

If you are looking for a bird from the latter category, the first ten days of June are your best bet. Present in Holland this year at the beginning of June: Griffon Vulture, Scops Owl, Baillon's Crake, Collared Pratincole, Red-breasted Flycatcher and River Warbler.
Not all birds stay around for a long time and some birds can only be heard, but then: we still have 5 more days before the first 10 days of June are over....