How do you go about asking your advisor/research assistant supervisor for a week off? Spring break is coming up and I plan on taking that week off to go traveling in Europe.  Do I ask my supervisor/advisor for permission or do I just inform them that I am doing so?  

Do I do it via email or face to face?  

Thanks!
Your contract should include some official vacation time. Doesn't it?

So, it should be your right to take those days off, and your supervisor shouldn't be able to say no, unless it interferes with your work, such as lectures, lab schedules, ...

But it is still nice to keep it cool and ask politely, better in person.