My children and other relatives have over the past few years had a variety of internships. Some have been willing to let their interns take a week off (they had PTO hours), others were neutral, and some were upfront about not allowing any vacation.
The one that expected interns to be there every workday said so in the position description that was visible before they could even apply. The company even mentioned it during the interview.
One type of internship that did limit the ability to miss days was one where the intern needed to complete X hours in order to receive the college credit. They has no problem with small schedule adjustments, but there was no point in missing enough days to fail to meet the program requirements. The position was unpaid, and you just paid money to the university to fail a class.
It is fine to ask.
If it is a deal breaker: if they say no you will not want the internship; then ask during the interview.
If it isn't a deal breaker: bring it up after you accept, but before you start,
In either scenario don't bring it up in a rude manor, but bring it up in a positive light. You want them to know you are helping others, not wanting a week off for beach week. It is best to at some point get it in writing (via email) so that they don't forget.