Map from Google - Belarus

In Belarus the climate is continental, with cold winters and relatively warm summers. The continentality is progressively accentuated as we move away from the sea, and we enter into the great plains of Eastern Europe. The country is nevertheless exposed to different air masses, Russian or Arctic, Atlantic or Mediterranean, so it can experience sudden changes in temperature and weather.
Here are the average temperatures of Minsk, the capital city, located in the centre of the country.
Average temperatures - Minsk
Min (°C)-7-7-33812131384-1-6
Max (°C)-2-14121922242317103-1
Min (°F)191927374654555546393021
Max (°F)283039546672757363503730

Precipitation amounts to around 700 millimetres (27.5 in) per annum, and it is well distributed throughout the year, although it is more abundant in summer, due to afternoon thunderstorms.
Here is the average precipitation.
Average precipitation - Minsk
Prec. (mm)403540406085907060505055675

Winter in Belarus is cold, with temperatures normally below freezing (0 °C or 32 °F) even during the day for a few months. The western cities (see Brest, Grodno) are less cold, and have an average temperature in January of about -3 °C (27 °F), while proceeding east the average reaches -4.5 °C (24 °F) in Minsk, to nearly -7 °C (19 °F) in eastern areas.
The snow cover in the eastern part of the country lasts about a month longer than in the western part: about four months instead of three.
The coldest periods are those in which the Siberian Anticyclone reach the country: it seemed to have disappeared in the early 2000s, but has been returning in recent winters. During these periods, the temperature can drop to -30 °C (-22 °F), but in the worst winters of the past century (when the average winter temperatures were a few degrees lower than today) they could drop even to -40 °C (-40 °F).

In spring, the temperatures increase rapidly. The thaw occurs between March and April, which is also a difficult time for travelling because of the mud, at least outside the cities. At the beginning of April there may still be ice and snow, while at the end of the month the first warm days can occur, with more than 25 °C (77 °F).

Summer in Belarus is quite warm, and unlike in winter, it tends to become progressively warmer as you move from the north-west to the south-east. In fact, the maximum temperatures range from 20/21 °C (68/70 °F) in the north-western cities, to 23/24 °C (73/75 °F) in the central areas (see Minsk), to 24/25 °C (75/77 °F) in the south-east.
During summer, hot periods, when highs can get closer to 30 °C (86 °F), alternate with cooler periods, which are dominated by the cool and humid Atlantic currents. Despite some frontal systems of Atlantic origin, in this season the sunshine amount is acceptable.

Autumn is a short season, when the temperature decreases rapidly, and the weather soon becomes cloudy, rainy and foggy. The first snowfalls usually occur in November, and sometimes even in October.

When to go

The best time to visit Belarus is summer, from June to August. The days are very long, the temperatures are mild, and at times even hot during the day, while nights remain cool. In hot and muggy days, some thunderstorms, especially in the afternoon or in the evening, can develop.
Certainly, the country may have its charms even in winter, in which case the month of February will be preferable, because it has longer days, even though it's still cold and snowy.

What to pack

In winter: very warm clothes, fleece, parka, hat, gloves, scarf, warm boots.
In summer: light clothes, T-shirts and shorts, but also long pants, jacket and sweater for the evening and cool days; raincoat or umbrella.