Warsaw experiences cold winters and warm summers, the winters also seeing a fair amount of snow.  Summer is the best time to visit the city, with temperatures averaging in the 80's, but with low relative humidity.  Summertime in Warsaw does see more precipitation than the rest of the year, sometimes it rains heavily.  For a daily weather forecast of Warsaw and to view its yearly average conditions, visit BBC Weather

Summers can get very hot, therefore the best time is May, or early autumn. And if going June to August, make sure you have an adequate sun protection and have a bottle of water handy.