The reason people get sick when they have time off (You'll notice this most in students) is because they can actually relax and have the time to get sick. Until then, your body is running on stress and adrenaline, which is weakening your immune system, but keeping you going. During this time you're exposed to all kinds of airborne crap (especially if you're a student!) and it just waits until your body relaxes. Then you get sick.
If you notice, ALOT of students tend to get sick right after finals or other huge, important exams. A few unfortunate individuals get sick right before, or during, but the bulk get sick almost immediately after.