"Ble dych chi'n mynd dros y penwythnos?"
Translation:Where are you going over the weekend?
It does. There are two issues here. One is that there are two types of 'over' One is the location - 'there is a roof over my head' and the other is the crossing - 'she walked over the hill' - i.e. from one side to the other. Dros means 'over' only in this 'crossing' sense as it is related to words like trans 'across' in Latin and English words like transport, all of which are to do with getting to the other side, or being on the other side.
The second issue is that we think of the 'weekend' as a period of time - so at or on would be logical. But historically, it was the boundary between one week and the next, so we often say over the weekend as this originally meant 'crossing from one week into the next'. D
Curiously, I got this as a multiple choice question, and one of the choices was bythefnos, which would be grammatically correct: "what are you doing over the (next) fifteen days?" I'm not sure how Duo picks the wrong answers in these questions, but clearly they can mess up every now and again...
Duo uses a secret algorithm that chooses words at random that do sometimes fit. What makes it worse is that the writers and mods do not get to proof-read the choices. Even worse, they cannot tell from this page that it is a multiple choice - except that you clarified it yourself. So you often get a conversation that goes
What is wrong with bythefnos?
It doesn't mean 'weekend'.
No one said it meant 'weekend', but it does fit.
Was it a multiple choice?
We'll fix it. Please always state that it was a multiple choice in future.
I've seen loads of these in the Gaelic course, and had at least a dozen corrected. It can get quite fiddly if the grammar does not allow all options. For example, in a particular example, the answer they wanted was loch and the grammar showed it was singular. They also offered lochan which can mean 'lochs' (so would not fit) but it also means 'wee loch', so it would fit. It can be quite unfair as sometimes the grammar required to know that something does not fit is grammar that has not yet been covered.
In this case, I had to check the gender of pythefnos. It is, according to the Ap, gender fluid, so bythefnos should be accepted, even though some people would say pythefnos.