I was wrong when I wondered if it was in Poznan...
I thought that at first.There is a Museum of Historical Costume there, but these dresses looked too new to be a part of that.
But where?
There seems to be wedding fairs held all over Poland e.g at Targi Kielce amongst other venues:
targikielce.pl/en/about-us/news/fashionable-wedding-expo-on-the-horison,8136
I'm not so sure it's that though. There is also a huge wedding fair that takes place in Warsaw.