"Where did we eat on Sunday?"
Translation:Onde nós comemos no domingo?
Have a look at [http://tools.verbix.com/webverbix/Portuguese.html?verb=comer]
You will see that "comemos" is both used for preterite and present, so both "do" or "did" should be accepted.
DuoLingo does this a lot, throwing new constructions into the exercises without introducing them. It initially seems confusing, if you're used to a straightforward, class-like language instruction, but it emulates the way natural language acquisition happens more closely.
I've worked through all of one language tree and most of a second, and I'm solidly convinced that this approach actually makes DuoLingo much more effective. Not only does it introduce more advanced concepts earlier on, but it makes it more likely that you'll connect the dots with concepts like this, in your head, in such a way that makes you better able to understand and converse with native speakers.
From my memories of when I lived in Brazil, this would have been the natural answer, (except it would prbably have been "Onde foi que ...) but I guess this is classed as a colloquialism rather than strictly grammatical. The difference between Duolingo language and real language!