Map from Google - Netherlands

The climate of the Netherlands is influenced by the North Sea and the Atlantic Ocean, so it's cool, cloudy and humid for most of the year. The rains are not so abundant, ranging between 750/850 millimetres (29/33 inches) per year, but they are frequent throughout the year.
Here is the average precipitation in the capital Amsterdam.
Average precipitation - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDecYear
AmsterdamJFMAMJJASONDY
Prec. (mm)654560505060707075809080795
Prec.(in)2.61.82.4222.42.82.833.13.53.131.3
Days1310910910111111121212130

The amount of sunshine is never excellent, because the sky is often cloudy even in summer, however, the sunniest months are May and June.
The wind blows frequently, especially in autumn and winter, when there may be even storms.
Being a small and flat country, the Netherlands have a quite uniform climate, although there's a slightly greater continentality in inland areas, compared with coastal areas, which in turn are more windy. For this reason, it can be said that the various Dutch cities (Amsterdam, Rotterdam, Utrecht, The Hague, Eindhoven, etc...) have essentially the same climate.
Here are the average temperatures of Amsterdam.
Average temperatures - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDec
AmsterdamJFMAMJJASOND
Min (°C)0024810121210741
Max (°C)5581216192121181496
Min (°F)323236394650545450453934
Max (°F)414146546166707064574843

Winter, from December to February, is cold but not freezing, with daily average temperatures around 2/3 °C (36/37 °F), that is, around freezing (0 °C or 32 °F) at night, and around 4/5 °C (39/41 °F) during the day.
There are periods in which Atlantic currents prevail, and the weather is mild, with highs above 10 °C (50 °F), but windy and rainy as well; in other periods, when the country is reached by high pressure systems, the weather can be dull and foggy.
On the contrary, cold air masses from Eastern Europe may sometimes reach the country, bringing cold spells, and in these cases the temperatures can drop below freezing even during the day. This kind of frosty weather typically lasts a few days, and is followed by the return of the westerlies, which may initially bring some snowfalls; therefore, snow is generally not abundant, and is often followed by rain.
The winter landscape covered with snow, with frozen canals, which can be seen in the paintings of Flemish artists, relate to a period (the so-called Little Ice Age, roughly 1450 to 1850), in which Europe's climate was colder than today. In recent times, situations of this type occur once every few years and have a shorter duration; however, when this happens, the Dutch are happy to play their national sport, ice skating, outdoors.

Pieter Bruegel the Elder, hunters in the snow - 1565

Spring, from March to May, is a cool season, in which the temperature slowly increases. In April, cold days are still frequent, with highs around 10 °C (50 °F), but this kind of weather can occur until the first half of May. In May, the average maximum temperature is still around 16 °C (61 °F). However, starting from this month, the expansion of the Azores High can bring the first sunny and relatively warm days, especially in the second half.

The Dutch summer, admittedly, is not the best. From June to August, temperatures are generally pleasant, in fact the daily average is around 17/18 °C (63/64 °F). Cool and rainy days are quite frequent, with maximum temperatures below 20 °C (68 °F), however, they alternate with warmer days, in which the sun peeps through the clouds, and on early afternoon the temperature can exceed 25 °C (77 °F). In short, the Dutch summer is similar to that of London. On average, the maximum temperature fluctuates around 21/22 °C (70/72 °F) in much of the country, and only around 20 °C (68 °F) on the coast; normally, the temperature reaches 30/32 °C (86/90 °F) only on two or three days per month.

Autumn, from September to November, is initially pleasant in September, especially in the first half, while later it becomes gradually colder, with cloudy and rainy days, and also windy.

Netherlands

The brave, who wish to swim in the sea, must know that the North Sea is very cool even in summer: the water temperature reaches 16 °C (61 °F) in July, and 17 °C (63 °F) in August. In addition, along the coast a cool breeze blows, often too cool, at least for those who are not accustomed to it, or don't come from Nordic countries.
Sea temperature - Amsterdam
AmsterdamJanFebMarAprMayJunJulAugSepOctNovDec
AmsterdamJFMAMJJASOND
Sea (°C)87891113161717151310
Sea (°F)464546485255616363595550

When to go

The best time to visit the Netherland runs from mid-May to mid-September, when the temperature is usually mild or pleasantly warm, the rains are quite frequent, but sometimes even the sun comes out. May and June are a bit cooler than July and August, but at this time the country is in bloom; in the first half of May, which can still be a bit cold, you can admire fields full of tulips, which are in bloom between mid-April and mid-May.
In the first half of September, maximum temperatures are still around 20 °C (68 °F).

What to pack

In winter: warm clothing, sweater, coat, wind jacket, umbrella, possibly warm hat and gloves for colder periods.
In summer: spring/autumn clothes: t-shirt for warm days, shirt, jacket, sweatshirt or sweater; raincoat or umbrella.