There isn't a good answer to "When is the weather nice in Holland?"   There is a persistence of cold and rain.  This occasionally gets replaced by oppressive heat--oppressive mainly for the fact that A/C is virtually non-existent.  This isn't to say that there aren't a few good days, but they're unpredictable and sporadic. 

Wear layers and bring something waterproof.  While the rain isn't as bad as some would suggest (or at least hasn't been this year), you might want to make it a habit to almost always have an umbrella handy, as showers can come on suddenly.  Also, as suggested above, it isn't unusual to have a couple of very warm days followed by chilly, windy weather.  The Dutch say that there's never bad weather, only bad preparation. 

For those wanting more detail, here goes: from January to late-April, it is consistently in the 40s(F) with rain coming mostly during the evening and night.   In this case consistently means EVERY SINGLE DAY.   After that, it can alternate between hot and cold.

A couple of quotes from locals inclue: “Last year we didn’t really have a summer,” and “Well, we’ve probably seen the best days of summer already.” (spoken June 23rd!)  Your best bet is to not plan a trip exclusively focused on outdoor activities.  Scheveningen is great (in a Coney-island sort of way) but there are far more winter jackets there than bikinis.  There are lots of great museums and places to shop, and a smart traveler will have a few as backup.