Yes, it may affect some tourist attractions and also banks and other public facilities. My experience in being there on that date is that some things were closed and others were open. So, it depends on what you want to see. I would recommend looking at the websites from some of the things you want to see (many of them will be in English as well as Polish). You can certainly walk around town and some restaurants will be open. It is a big religious holiday as well as a public holiday. August 15th is also Armed Forces Day in Poland and so when I was in Krakow, there was a big parade with people in all branches of the armed forces, some on horseback. It was quite impressive.