DIY stores that are open on Sundays

My partner says that usually these types of places are open on the first Sunday of every month. We just gave a call to the Obi in Wedding and no answer so I'm assuming they're shut and their website doesn't list them as open on Sundays either. Anyone else have an idea? (I gotta shut down this argument between him and me! )

Check the following link. He's wrong. There are a few Verkaufsoffenesonntage in the year plus extra ones in the run up to Christmas.
Berlin (along with other Länder) started relaxing shop opening hours in 2007 but Sundays remain pretty much sacrosanct.

http://www.extern.berlin.ihk24.de/servicem.../2sonderoef.jsp