Translation:The day before yesterday, we learned what had happened in our bedroom.
They may want to enforce the proper sequence of tenses here. The Czech puts the action in the subordinate clause in the past with respect to the main clause. In English the past perfect is used for that, although colloquially it's very common to use a relaxed sequence of tenses when one need not be precise in order to be understood.
Surely, 'the day before yesterday we got to know etc...' is correct? I disagree 100% with earlier comments, particularly with Petr's assertion. An educated native English-speaker would DEFINITELY have no problems with 'got to know' here! Are we students studying English or Czech?