If a person vows to refrain from an act for a specific period, then once the period comes to an end, he can do the act. If before the period comes to an end he does the act owing to forgetfulness or necessity, then nothing is obligatory for him [concerning this matter]; however, he must still not do the act [again] until the period comes to an end. In the event that he does the act again without a legitimate excuse before the period comes to an end, he must give kaffārah.