In Parshas Pekudei, the final parshah in which the Torah discusses the building of the Mishkan and its vessels, the pesukim detail all of the items that they had made and that they were brought to Moshe (Shemos 39:33-43). In the following perek, it is written that Moshe erected the Mishkan.
In Parshas Naso Rashi (7:1) says that during the seven days of miluim, Moshe would erect and dissemble the Mishkan every day. Erecting the Mishkan involved putting up the walls and placing the covering on top, as a roof. The Panim Yafos asks: since invariably one of the seven days of miluim must have been Shabbos, how was Moshe able to erect the Mishkan on Shabbos? This should be a prohibition of the melachah of boneh (building). We learned in last week’s parshah (Parshas Vayakhel) that the building of the Mishkan was forbidden on Shabbos.
The Panim Yafos answers that several people erected the Mishkan. As such, it was not forbidden on Shabbos under the halacha of shenayim she’asahu (more than one person performing an action). However this answer is not very understandable. The halacha of shenayim she’asahu does not apply when more than one person was needed for that particular action. For example, if two people are needed to lift and carry an item, we will not apply the halacha of shenayim she’asahu if two people carry the item in reshus harabim. We only apply the halacha of shenayim she’asahu when only one person was needed to perform the action. Since the Mishkan was extremely heavy, it is unlikely that it could have been performed by only one person.
The Chasam Sofer (Teshuvos Orach Chaim 72) suggests another answer. He discusses whether a structure that will be taken down in a short period of time is permitted to be erected on Shabbos. He quotes a Yerushalmi, in the seventh perek of Shabbos, that questions how we can hold that building is forbidden on Shabbos from the Mishkan when the Mishkan was a temporary structure that was constantly erected and dissembled. The Yerushalmi answers that the Mishkan was not considered a temporary structure because the Bnei Yisrael encamped and traveled by the word of Hashem.
The Chasam Sofer adds that this was not the case during the seven days of miluim because then, the Mishkan was not erected by the word of Hashem. As such, the Yerushalmi’s answer would not apply to the seven days of miluim. The Chasam Sofer says that since it was a temporary structure during the seven days of miluim, Moshe was permitted to erect the Mishkan on Shabbos.
A machlokes regarding tefillin is whether the knot of the tefillin shel yad must be tied every day anew or just tightened (as is the prevalent minhag). Rabbeinu Eliyahu cited in Tosafos (Menachos 35b and Chullin 8a) that says the knot must be tied each day, for it is written in a pasuk: “u’keshartem” (and you must tie them).
Rabbeinu Tam disagreed with this, saying that if one unties the knot every day, the knot will not be considered a kesher shel kayama– a permanent knot. Since it is not a kesher shel kayama, we cannot consider it a knot; thus it is not a fulfillment of the mitzvah of u’keshartem.
The Avnei Nezer (Orach Chaim 183) suggests that Rabbeinu Eliyahu would answer Rabbeinu Tam’s question by stating that since one is performing a mitzvah by tying the knot, the mitzvah identifies that the knot will be defined as a kesher shel kayama. Since it is being used to perform a mitzvah, it is recognized as a knot.