Poland has 4 main seasons plus there is something called "przedwiośnie" - early spring in March.  

Summers are usually wet with scattered showers and storms. They are warm and sometimes hot (usually from 20C up to 35C). Summer in Poland lasts from June to early September. The hottest places are the Lower Silesia and Małopolska Provinces, the seacoast area is usually a bit colder with more rainfall. June and July are a high tourist season because of the national school break. Early September (when students are back at school) is a good time to visit Poland. 

Spring (from late April to June) is probably the best time to visit Poland. Rains and heat-waves are less frequent than in summer. April is usually warm - from 15 to 20C - May and June can be warmer. 

Autumn weather is very unpredictable. Usually late September and October are warm with possible showers. November is frequently very wet and can be cold but on the other hand - it can sometimes be just lovely, sunny and warm presenting the so-called "Polish golden autumn" (złota polska jesień). In late November snow is possible, especially in the mountains.

Winters are cold with possible heavy snowfalls, especially in the mountains and eastern Poland. The temperatures from December to late February can be as low as -20C (or even lower) but usually fluctuate around 0C to -10 C. Winds are strong and when temperatures are above zero centigrade then wet snow or rain is possible. It's the high tourist season in mountain resorts. 

To sum up - Poland can be visited all year round but the best weather for sightseeing and traveling around the country is from April to June and from September to early October. July and August are also fine but touristy places can be more crowded then.

***The above mentioned descriptions are very general. Polish weather is usually hard to predict so if you are planning a visit check the forecasts a day or two before your departure - and pack your luggage accordingly.***

Packing tips: taking an umbrella is always a good idea, you can also buy cheap ones in stores all around the country (10-20 PLN). From April until October it's good to take a waterproof jacket + a warm sweater with you, from November to March it's better to take some warmer waterproof coat. Always check weather forecasts before packing!