it's funny to see what everyone says about this...i'm diong a PI right now and extending until January. I'm getting married in September and when I asked for time off (like half of september) the general manager of my area told me that as interns, it's pretty sweet because we can take off whenever we want....i mean, you don't get paid for it or anything....
SOOO --- I would talk to your Leader and let them know ahead of time. Some places, like my area, we can move our days off around, and I'm allowed to put 2 weekends together instead of actually taking time off...just working like Sunday-Thursday, then off Friday, Saturday, Sunday, Monday, and then working Tuesday-Saturday.
Just some thoughts.