Map from Google - Belarus

In Belarus the climate is continental, with cold winters and relatively warm summers, and two short intermediate seasons. The degree of continentality increases from northwest to southeast, that is, as one moves away from the sea, and enters the great plains of Eastern Europe.
However, the country is 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
MinskJanFebMarAprMayJunJulAugSepOctNovDec
MinskJFMAMJJASOND
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
MinskJanFebMarAprMayJunJulAugSepOctNovDecYear
MinskJFMAMJJASONDY
Prec. (mm)403540406085907060505055675
Prec.(in)1.61.41.61.62.43.33.52.82.4222.226.6
Days109109911119991112119

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 towards the east the average reaches -4.5 °C (24 °F) in Minsk, to nearly -7 °C (19 °F) in eastern areas.
In the eastern part of the country, the snow cover 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 reaches the country: in the early 2000s it seemed to have disappeared, but it has returned 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 temperatures were a few degrees lower than today, especially in winter) they could drop to as low as -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 and on unpaved roads. While on early April there may still be ice and snow, at the end of the month the first warm days can occur, with temperatures above 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 approach 30 °C (86 °F), alternate with cooler periods, in which the cool and humid Atlantic currents prevail.
Despite some frontal systems of Atlantic origin, in this season the sunshine amount is acceptable.

Autumn is a short season, in which the temperature decreases rapidly, and the weather quickly 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. During 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-shirt and shorts, but also long pants, jacket and sweatshirt for the evening and for cool days; raincoat or umbrella.