Saturday. Well Friday after I get home is even nicer than Saturday, and it's a short day as well (school ends at 11:20!), but it doesn't beat a whole day of spare time.
I don't like Sunday that much. The thought of the next week being close is stressful, and I also do homework on Sundays, so I generally don't like Sunday that much. Plus I go to bed at a fixed time on Sunday, which makes it slightly depressing to lie in the bed after a whole weekend and think of the next schoolweek.
Oh, and Rebecca Black ruined Friday. So Saturday. jk