Translation:Where are you going over the holidays?
In my opinion, "for the holidays" and "over the holidays" have slightly different meanings. The former suggests you expect someone to spend their whole holiday period somewhere away from home, whereas the latter could be any amount of time during the holidays. Imagine if the sentence was "Where are you going over the next few months" - then we couldn't replace "over" with "for" and get the same meaning. Either way, the sentence above is grammatically correct.
Try to be consistent, Duo! I translated "dros" as "over" the weekend. Wrong! Should be "for". So, wen "dros" came up again in "dros y gwyliau" I translated it as "for". Wrong again, should be "over". As far as I'm concerned, I can see nothing wrong with either translation but, over and above that, I find it intensely irritating that Duo makes an arbitrary decision and then tells me I'm wrong because I did not make the same arbitrary choice between two perfectly acceptable answers.