That would be 'yn ystod y gwyliau' and would be used in different sentences but there is clearly some overlap here so I'll add it as an alternative in English. Thanks for the suggestion.
Could you provide a couple of example sentences for both phrases, please?
I would tend to use 'dros y gwyliau' to talk about a one off past event.
eg "dros y gwyliau es i i Ffrainc' = Over the holidays I went to France
while I would use 'yn ystod y gwyliau' for some past continuous action, ie the imperfect
'yn ystod y gwyliau ro'n i'n gweithio yn y siop' = During the holidays I was working in the shop
But you could swap the sentences and still make sense.