Poland's climate is moderate and generally has equal periods of cold or warm weather. June and July are one of the warmest and most beautiful months. However be advised that July can be rainy. The current weather forecast is available here.
So in the summer, it is good to bring sunglasses, a sunscreen and an umbrella.
When planning to visit the Tatra Mountains or the Salt Mine, please take your jacket/fleece/sweater and hiking shoes.