I don't have my contract on me but as it's been explained to me, and I didn't get any grief over it when I did it in July for the 4th, I was paid all my vacation time + the holiday and could choose to take the following Monday off, without penalty aside from getting a paycheck for 4 days worked instead of 5 since I got paid for 6 because of holiday+vacation already.
As others have said though, check your supplementary, or tell us where you're at so if someone else in that area knows they can speak up.