Map from Google - Belgium

The climate of Belgium is cool and wet, being influenced by the Atlantic Ocean, especially on the coast and the plain, while it becomes more continental and even more unstable in summer in the south-eastern regions (Ardennes), which are located at higher altitudes, in addition to a greater distance from the sea.

Precipitation is frequent, but not particularly abundant. The coastal area is less rainy and is also slightly more sunny than the inland area: on the coast, in Ostend, 750 millimetres (29.5 inches) of rain per year fall; in Brussels, a hundred kilometres (60 miles) away from the coast, 820 mm (32 in) of rain fall; while in Spa, in the Ardennes, the rainfall amount reaches 1,100 mm (43 in).
Here is the average precipitation in Brussels.
Average precipitation - Brussels
BrusselsJanFebMarAprMayJunJulAugSepOctNovDecYear
BrusselsJFMAMJJASONDY
Prec. (mm)706070506575707565707080820
Prec.(in)2.82.42.822.632.832.62.82.83.132.3
Days131113101110101010111213132

Winter in Belgium is cold on the coast, but not icy. The average temperature in February (which on the coast is, albeit slightly, the coldest month) is about 3.5 °C (38.5 °F). Sunshine is rare and the rains are frequent.
Proceeding towards the interior, the temperature gradually decreases. Even here the rains are frequent.
In the Ardennes, the winter is colder, both for the altitude and for the greater distance from the sea: in Bastogne, 500 metres (1,600 feet) above sea level, the average January temperature drops to 0 °C (32 °F), and instead of rain, snow often falls.
Here are the average temperatures of Bastogne.
Average temperatures - Bastogne
BastogneJanFebMarAprMayJunJulAugSepOctNovDec
BastogneJFMAMJJASOND
Min (°C)-2-202691111851-1
Max (°C)2471116192020171263
Min (°F)282832364348525246413430
Max (°F)363945526166686863544337

Apart from the average temperatures, in winter the weather in Belgium varies depending on the meteorological conditions. The coldest days occur when the normal flow of the Atlantic winds stops, and is replaced by cold air masses from Eastern Europe: in these cases the temperature drops below freezing (0 °C or 32 °F), and snow can fall throughout the country. On the contrary, when the westerlies get stronger, the temperature can be mild even in winter, above 10 °C (50 °F), but sometimes there can be strong winds and storm surges too.
At other times, it happens that a high pressure system moves over the country, a phenomenon which is quite rare but not impossible, and in these cases the weather becomes cold and gray, with possible fog and low clouds.

Summer in Belgium is pretty cool on the coast, with maximum temperatures reaching 20 °C (68 °F) in July and 21 °C (70 °F) in August. There is no shortage of clouds and rains, due to the Atlantic fronts, which even in this season may reach the country.
In inland areas, the weather in summer is slightly warmer: in Brussels the July daily average is about 18.5 °C (65.5 °F), with maxima about 23 °C (73 °F), but even here hot days are rare. The rains are more frequent, because of the higher chance of thunderstorms in the afternoon.
Here are the average temperatures of Brussels.
Average temperatures - Brussels
BrusselsJanFebMarAprMayJunJulAugSepOctNovDec
BrusselsJFMAMJJASOND
Min (°C)1136912141411842
Max (°C)671014182123231915106
Min (°F)343437434854575752463936
Max (°F)434550576470737366595043

In the Ardennes, the temperatures are slightly lower than in the plains. Anyway, afternoon thunderstorms here are even more frequent.
Hot days, with maximum temperatures equal or greater than 30 °C (86 °F), are quite rare but not impossible.

Spring and autumn are cool and cloudy. It can be still cold in March and the first half of April, sometimes even in early May, with highs around 10 °C (50 °F). Spring, however, is less rainy than autumn and it's in fact the (relatively) less rainy season of the year. Autumn can be windy and gray, especially on the coast.

Dinant

In mid-summer (July-August) only the brave can try to swim in the sea: the sea temperature is 16 °C (61 °F) in July and 17 °C (63 °F) in August.
Sea temperature - Ostend
OstendJanFebMarAprMayJunJulAugSepOctNovDec
OstendJFMAMJJASOND
Sea (°C)87891113161717151310
Sea (°F)464546485255616363595550

When to go

The best time to visit Belgium goes from mid-May to mid-September, when the temperatures are mild, the sky is often cloudy, but the percent of sunshine is not so low. The mid-summer period (July-August) is relatively warm, but it's also the rainiest, because of afternoon thunderstorms which can form especially in inland areas.
The other seasons are not definitely inadvisable, because weather extremes in Belgium are rare, but late autumn (see November) is dark and rainy, while winters can at times be very cold or windy.

What to pack

In winter: warm clothing, sweater, coat, wind jacket, umbrella, possibly a warm hat and gloves for colder periods.
In summer: light clothes, but also some clothes for spring and autumn: shirt, jacket, sweatshirt or sweater; raincoat or umbrella.