The vast majority are September to June. There may be in company work during summer, but jobs like this aren't too common, as companies will also tend to have a quiet August as it's holiday time in many parts of Europe, including Poland.
A lot depends on how much business they have. The school where i work has a lot of business contracts, so these carry on during the summer, as do some of the EFS run courses. We offer summer courses but they are often cheaper than usual ones. They are also a bit thin on the ground, so many of the NS get summer work in UK.