Weather conditions in June (averages between 1981-2010):
- Min – Mean – Max Temperature: 11.9°C – 16.2°C – 20.6°C
- Days with rain: 15
- Amount of precipitation: 71.8 mm
More info here (in Dutch).
Light clothing during the day, but provide a (thick) sweater for in the morning and evening. It usually stays dry, yet be prepared for rain (showers) thus bring an umbrella or raincoat.
In any case, check the weather forecast just before packing and leaving …