If you are falling behind for the first time, your landlord deserves to know why. Being upfront might give you more flexibility in getting your rent caught up as you wait for the new paychecks to arrive in late September. It is not easy to admit that you are having trouble honoring your lease agreement, but at least letting the landlord know that you both intend to repay the arrears and already have the new job lined up could give you the time you need.