> 
> Ich möchte wieder in dem Gasthaus einen Kaffee trinken, in \_\_\_\_\_ wir samstags immer gegangen sind.
> 
> 
> 

I think it is ***das*** because the antecedent is ***dem Gasthaus*** which original form is *das Gasthaus*. So it translates to "which we always went (into) on Saturdays"?  

Is my reasoning correct?