-6C = 21.2 Farenheit
In Poland (I'm not sure if this information is accurate. I read it off of a website.) you are not allowed to wear spaghetti straps or shorts inside churches, so steer away from that. besides, it will be too cold to wear those anyway. I suggest you wear jeans, sweaters, tees, etc. just make sure you bring a warm winter coat too! if you have boot (or any other comfortable shoes) that would be helpful because you will probably be doing a lot of walking.
Oh, in case you're not familiar with how cold -6C is..it converts to 21F