There is no way to guarantee the arrival date. Even if the participant sends the day after the swap ends, it still may not reach you by the arrival date, since the sender has no control on what happens after the the post office has it. Having a sent by date at least guarantees to have them all in the mail by a certain time. You could require that everyone use Priority Main to send to you. I did not require it on mine but listed it as the prefered method.
