Map from Google - Poland

In Poland the climate is continental, with very cold winters, often below freezing (0 °C or 32 °F), and warm summers. The climate is milder along the northern coast, overlooking the Baltic Sea, while it becomes progressively more continental going to the south, where the distance from the sea is greater.
Here is the average temperature of Gdansk, located on the northern coast.
Average temperatures - Gdansk
GdanskJanFebMarAprMayJunJulAugSepOctNovDec
GdanskJFMAMJJASOND
Min (°C)-3-3-1371113131062-2
Max (°C)1261016192121171263
Min (°F)272730374552555550433628
Max (°F)343643506166707063544337

As mentioned, in inland areas the climate becomes slightly more continental, as we can see from the average temperatures of Warsaw.
Average temperatures - Warsaw
WarsawJanFebMarAprMayJunJulAugSepOctNovDec
WarsawJFMAMJJASOND
Min (°C)-5-4-138111312940-3
Max (°C)0271319222423181262
Min (°F)232530374652555448393227
Max (°F)323645556672757364544336

Precipitation in Warsaw is not abundant, given that it amounts to about 550 mm (21.5 in) per year; the driest seasons are winter and spring, while the rainiest is summer. In the rest of the country, precipitation follows a similar pattern. Here is average precipitation in Warsaw.
Average precipitation - Warsaw
WarsawJanFebMarAprMayJunJulAugSepOctNovDecYear
WarsawJFMAMJJASONDY
Prec. (mm)253025354570906545403045545
Prec.(in)11.211.41.82.83.52.61.81.61.21.821.5
Days875778118868992

Winter, from December to February, is cold throughout the country: the average temperatures in January is around freezing along the Baltic coast, and progressively decreases towards the south and east, ranging from an average of about -1 °C (30 °F) in the north-western area, to -4 °C (25 °F) in easternmost area, on the border with Belarus and Lithuania, to -3 °C (27 °F) in the southern area. In Warsaw, the capital, located near the centre of the country, the average in January is -2.5 °C (27.5 °F). The ground is often covered with snow from December to mid-March, although snowfall is generally not abundant. However, the weather during winter can have different characteristics: the sky is often gray, the temperature is around or a few degrees below freezing, and a light snow falls, but when cold air masses from Russia reach the country, the temperature decreases by several degrees, dropping below -20 °C (-4 °F). The coldest records are around -25/-30 °C (-13/-22 °F) in the interior, therefore in much of Poland, while they are around -18/-20 °C (0/-4 °F) along the northern coast.
Spring, from March to May, is initially cold, and in early April there can be late cold waves with snow showers; in April and early May, cold days alternate with the first warm days, while from mid-May the temperature becomes steadily pleasant, with highs around 15/20 °C (59/68 °F), and sometimes above.
Summer, from June to August, is mild or pleasantly warm: the average daily temperatures are around 17/18 °C (63/64 °F). The Baltic coast, which during winter was the mildest zone, in summer becomes the coolest: here highs in July and August are around 20/21 °C (68/70 °F), while they gradually increase towards the south, and reach 23/24 °C (73/75 °F) in the centre and south of the country.
Summer is the rainiest season, because Atlantic disturbances arrive even in this season, besides thunderstorms in the afternoon may occur, more frequently in inland areas, and even more near the Sudetes and Tatra Mountains in the south. In July, it rains on average for 10 days on the north coast, 11 days in Warsaw and 12 in the south. However, there's no shortage of sunny days, which may be hot, especially when warm currents arrive from the south-east, so that the temperature may reach 30/35 °C (86/95 °F).
In autumn, from September to November, the temperatures decrease rapidly, but the first half of September is still pleasant. Afterwards, the weather turns cold and dull.

In southern Poland, where there are mountain ranges such as the Sudetes and the Tatras, the climate is naturally colder, with cold and snowy winters (but not much colder than in the lowlands, because of the temperature inversion, a phenomenon in which cold air stagnates in the lowest layers, typical of cold plains in winter), and cool summers, with several afternoon thunderstorms. In Zakopane, a popular tourist resort, at 850 metres (2,800 ft) above sea level, the average temperature is -5 °C (23 °F) in January, and 15 °C (59 °F) in July. At higher altitudes, at 2,000 metres (6,500 feet) above sea level, it ranges from -9 °C (16 °F) in January, to 7.5 °C (45.5 °F) in July.
On the slopes of these southern mountains and in the valleys, a warm dry wind, similar to the Foehn, can blow, the Halny, especially in early spring and in autumn; sometimes this wind can be strong and cause damage.
The highest peak in the country is Mount Rysy, 2,503 metres (8,212 feet) high, on the border with Slovakia.

Warsaw

The Baltic Sea is very cool even in summer, however, the water temperature reaches 18 °C (64 °F) in August.
Sea temperature - Gdansk
GdanskJanFebMarAprMayJunJulAugSepOctNovDec
GdanskJFMAMJJASOND
Sea (°C)43348131618151296
Sea (°F)393737394655616459544843

When to go

The best time to visit Poland is summer, and in any case the warm season, which runs from mid-May to mid-September. The temperatures are usually cool at night, and warm during the day, however, there can be cool and rainy days, but also hot and sunny days.
In the mountains, it's still possible to ski at Easter, while at Christmas it can be very cold, and the days are short. Even in February it's cold, but at least the days are a bit longer.

What to pack

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