The Torah in Parashat Behaalotekha tells of the korban pesach offering brought by Benei Yisrael a year after the Exodus from Egypt, and of the group of temei�im � people who were ritually impure and thus unable to participate in the sacrifice.  They approached Moshe to ask if they could offer the paschal sacrifice, and God informed Moshe that people who are tamei on Pesach and thus unable to offer the korban pesach should do so one month later, on the 14^th of Iyar (Pesach Sheni).

            The Sifrei, as well as the Gemara (Sukka 25), cites different views among the Tanna�im as to the identity of these people.  Rabbi Akiva identified the temei�im as Mishael and Eltzafan, Moshe�s cousins who removed the remains of Nadav and Avihu after they were consumed by fire in the Mishkan on the day of its inauguration.  Another view is taken by Rabbi Yishmael, who claimed that these people were assigned the task of transporting Yosef�s remains through the wilderness.  Rabbi Yitzchak, however, dismisses both views, noting that all these people could have completed the seven-day purification process in time for the korban pesach offering.  The people who approached Moshe must have become temei�im within seven days of the sacrifice, and thus Rabbi Yitzchak maintained that they had tended to a meit mitzva � a deceased person with no family members to bury him � and were not able to complete their purification before Pesach.

            Seemingly, these Tanna�im debate the question of whether one who had the opportunity to regain his status of purity before Pesach but failed to do so is entitled to offer the paschal sacrifice on Pesach Sheni.  Rabbi Yitzchak rejected the views of Rabbi Akiva and Rabbi Yishmael because the people they named would have been able to offer the sacrifice had they undergone the purification process in time.  Hence, they would not deserve the opportunity presented by Pesach Sheni.  According to Rabbi Yitzchak, then, one may offer the korban on Pesach Sheni only if he did not have the opportunity to become tahor before Pesach.  Rabbi Yishmael and Rabbi Akiva apparently disagreed, and maintained that anybody who was tamei on Pesach, even if this was because he neglected to become tahor in time, is given the opportunity offer the sacrifice on Pesach Sheni.  (This point is made by Rabbi Chaim Jachter in an article in Hadarom, 5765.)

            In truth, however, one might distinguish in this regard between Rabbi Akiva and Rabbi Yishmael.  According to Rabbi Yishmael, these people were the ones transporting Yosef�s coffin, and as the nation had been encamped in Sinai for ten months, it stands to reason that these men had no contact with the coffin during this period, and thus had ample opportunity to become tahor but failed to do so.  Accordingly, it would certainly appear that Rabbi Yishmael allows one to offer the sacrifice on Pesach Sheni even if it was negligence that caused him to be unfit for offering the sacrifice on Pesach.  Rabbi Akiva�s view, however, might be explained differently.  Malbim (both here in Parashat Behaalotekha and in the beginning of Parashat Shemini) writes that Rabbi Akiva and Rabbi Yitzchak�s debate may be understood as a question concerning the chronology of events.  Most commentators maintain that the seven-day milu�im process, whereby the Mishkan was formally consecrated, took place during the final week of Adar, and the eighth day, when Aharon and his sons began serving as kohanim, was Rosh Chodesh Nissan.  It was on this day when Nadav and Avihu died, and thus Mishael and Eltzafan became temei�im on Rosh Chodesh Nissan.  Hence, according to Rabbi Yitzchak, they had ample time to regain their status of purity before Pesach (the 14^th of Nissan), and would not have been given the opportunity to offer the sacrifice on Pesach Sheni.  Ibn Ezra (beginning of Parashat Shemini), however, maintained that the milu�im process began on Rosh Chodesh Nissan.  In his view, Nadav and Avihu perished not on the first of Nissan, but rather on the eighth of Nissan.  Malbim suggests that this was the position taken by Rabbi Akiva.  Mishael and Eltzafan, according to this view, became temei�im on the eighth of Nissan, and did not regain their status of purity until a week later � the night of the 15^th of Nissan.  Since they were still ritually impure at the time of the slaughtering of the korban on the afternoon of the 14^th of Nissan, they could not participate in the sacrifice.

            If so, then Rabbi Akiva accepts the premise that only those who did not have the opportunity to become tahor before Pesach are allowed to bring the offering on Pesach Sheni.  His disagreement with Rabbi Yitzchak concerns the chronology of events, as in his view, Mishael and Eltzafan were not able to regain their status of purity before Pesach.