You know what else will help? If on jump day everyone driving a car picks up their feet so that their cars will be lighter.
Although you can calculate a change, the calculation wouldn't prove jack.
1. It'd be incredibly inaccurate given all the millions and millions of variables that would need to be taken into account.
2. Even if we did do this, and the change could be measured, it would be utterly meaningless against all the other forces changing our world each day. The effect you did now would be wiped out within minutes by a tidal shift, mild earthquake, or polar-ice movement.
3. You'd have to repeat this task constantly for quite a while to build up enough change for it to have even an insignificant effect. This would be even harder, since you'd have to consider that for each jump, you're most likely being reset by the rest of the forces of the Earth. One step forward, one step back.
4. What happens when you do it too much? Given the incredibly small result of this action, how would you know it was working? You could hop all day and not see a single effect in even the sharpest of instruments.
5. Most vulnerable day? Is this the day the mole-men from the core exchange the supports holding the continents up?
GET REAL. GET A CLUE. THEN GET STUFFED.
Now if you'll excuse me, I've got to drive home in reverse so I can get back the time I spent driving here.
Jesus Christ in 4-oz cans...The things people will believe...