Literally the only reason this question even exists, really, is because people don't universally regard Sunday as the "first" day of the week. Even in places where it's "technically" the first day of the week, it often isn't regarded as such, because the work week supersedes it.
The answer to this question, in any case, is "in 6 days," though, because contextually it's the only answer that makes sense.
The answer to this question, in any case, is "in 6 days," though, because contextually it's the only answer that makes sense.