Eva – I went in the very beginning of December, which is the best time to see Christmas markets (they are open from the end of November to the 22-26 of December, depending on the location.) But as far as weather, summer is probably the most popular and also the most crowded. I went to Poland in April and it was a bit chilly – I’ve heard May is perfect (not too hot, not too many tourists, flowers have just popped open) but it all sort of depends if you dont mind rain, or want warm!