The Torah in Parashat Behaalotekha (9:1-14) tells of the group of people who were temei’im (ritually impure) at the time of the paschal offering, and were thus unable to offer the sacrifice at its proper time.  God commanded that they should instead offer the sacrifice one month later, on the 14^th of Iyar. 

            The Gemara in Masekhet Sukka (25a-b) famously identifies these temei’im as people who came in contact with a dead body for the sake of a mitzva.  One view identifies them as the bearers of Yosef’s coffin, which was taken with Benei Yisrael out of Egypt and brought to Eretz Yisrael for burial.  Another opinion is that these were Mishael and Eltzafan, Moshe’s cousins who were instructed to tend to the remains of Nadav and Avihu, Aharon’s sons who perished on the day of the Mishkan’s inauguration.  A third view cited in the Gemara claims that these temei’im had buried a meit mitzva – a deceased person who had no family members to tend to the burial.  According to all three opinions, these temei’im allowed themselves to contract tum’a despite knowing that this would prevent them from offering the korban pesach.  The Gemara thus views this episode as the Biblical source of the famous rule of oseik be-mitzva patur min ha-mitzva, which absolves one from a mitzva obligation while he is occupied with a different mitzva.  These temei’im were not required to take their looming korban pesach obligation into account when deciding whether to tend to a burial, thus demonstrating that one’s current involvement in a mitzva takes precedence over a mitzva that surfaces subsequently. 

            Among the many interesting issues that have been addressed concerning this halakha is whether or not it extends to mitzvot kiyumiyot – meaning, mitzvot that are not technically obligatory.  The Netivot (72:19), for example, raises this question in the context of gabba’ei tzedaka – the officers assigned over a communal charity fund.  Would they be exempt from other mitzvot while tending to their duties as the charity officials?  The Netivot rules that they would not receive the exemption of oseik be-mitzva, since there is no technical obligation to serve in this capacity.  It goes without saying that one who assumes responsibility for the communal charity performs a valuable and important mitzva by assisting the needy of his community.  However, in the view of the Netivot, since a gabbai tzedaka accepts this position voluntarily, and is under no personal halakhic obligation to take on this role, his charitable work does not exempt him from other mitzvot.  As such, even when he is involved in his work, he must break to attend to other mitzvot that present themselves. 

            Rav Meir Auerbach, in Imrei Bina (O.C. 13:3), disagrees with the Netivot’s ruling.  He notes that the Gemara in Masekhet Sukka (26a) applies the rule of oseik be-mitzva even to soferim who are busy writing Torah scrolls, tefillin and mezuzot.  Clearly, there is no personal obligation to take on the role of scribe, and yet soferim are granted an exemption from other mitzvot while they tend to their work.  This would seem to prove that the oseik be-mitzva exemption is not limited to those involved in strictly obligatory mitzvot. 

            An interesting observation is made by the Aderet (Rav Eliyahu David Rabinowitz Teomim), who noted that this issue might depend on the aforementioned debate concerning the identity of the temei’im who were unable to offer the korban pesach.  As mentioned, one view identifies these people as the bearers of Yosef’s ark.  Nowhere is there any indication that this job was specifically assigned to a certain group of people, and we might reasonably assume that the pole-bearers volunteered for this role.  (The same can be said about the third view, identifying the temei’im as people tending to a meit mitzva.)  Yet, despite the fact that they were not strictly obligated to perform this mitzva, their job yielded an exemption from the mitzva of korban pesach, seemingly proving that the oseik be-mitzva exemption applies even to voluntary mitzvot.  By contrast, Mishael and Eltzafan were specifically instructed by Moshe to tend to the remains of Nadav and Avihu (Vayikra 10:4).  If these were the temei’im who forfeited the mitzva of korban pesach, then we might limit the exemption to those involved in a mitzva in which they are strictly obligated. 

(Based on Umka De-parsha, Parashat Behaalotekha 5769