Starting in April you have Easter, a Big Celebration in Poland, then the flowers start blooming, the birds singing and the weather is great, summers are great, in August it can get hot sometimes, Novembers can be rainy, cold, miserable but that in itself has it's own beauty. Once the snow starts falling, the roads get bad, specially with so many drivers who are always in a hurry somewhere it can get nasty. They don't clean the roads as fast as in the US, but if you look beyond that, you can see beauty of winter and if you enjoy skiing or snowboarding, it's worth spending time in the mountains.
I recommend May through August. I like May/June best; not too hot and pretty Polish late spring/early summer. September feels a little like fall, October too. November it could be cold and even snow.