You must keep discs for 15 days plus the current week. So if for example you were stopped driving on Tuesday you would need to have the week one, the week two and the Monday. When you get to the end of week three hand in week one and start again. Even if you were stopped in the first ten minutes of driving you would still be able to show week one(that was two in the first part of the answer) week two (which would be the completed week from Monday, then the Tuesday you were stopped, and the remaining days of the week).
That equals fourteen days but of course there is one in the pot which gives you the fifteen and it is from the 'current week'.
Think it out.