The Rambam, in Hilchos Beis Habechirah 1:12, derives from the pasuk in this week’s parshah, “u’veyom hakim es haMishkan… – and on the day the Mishkan was set up…” (Bamidbar 9:15), that the Beis HaMikdash can only be built by day, not by night. Further in that halacha the Rambam writes that both men and women are obligated in the mitzvah of building the Beis HaMikdash. The Kesef Mishneh explains that the source for the halacha that women are obligated in this mitzvah is from the pasuk in parshas Vayakhel: “v’kol ishah chachmas lev beyada tavu – and every wise-hearted woman spun with her hands.”
The Achronim are bothered by this obvious question: Why are women obligated in this mitzvah? Since it only applies by day, it should fall under the category of mitzvos assei she’hazman gramma (time- sensitive mitzvos) that women are exempt from fulfilling?
The answer by some Achronim is based on the following Yerushalmi: The Yerushalmi Yoma 1:1 says that the mitzvah of building the Beis HaMikdash essentially applies even by night – except that if it is built at night it is not fit for the avodahs of the daytime. If the mitzvah only applied by day, a Beis HaMikdash that was built at night should not be fit for any avodah. This indicates that the mitzvah applies even by night; thus it is not a mitzvas assei she’hazman gramma, and women are obligated in it.
Another suggested answer is that the Rambam says in Sefer Hamitzvos (mitzvas assei 20) that the building of the Beis HaMikdash’s vessels is included in the mitzvah of building the Beis HaMikdash. The Aruch Laner, on Sukkah 41a, says that the vessels of the Beis HaMikdash can be built at night. Therefore the mitzvah of building the Beis HaMikdash applies by night as well. It is therefore not a mitzvas assei she’hazman gramma.
Even according to the Achronim who disagree with the Aruch Laner and hold that the vessels must be built by day (just as the Beis HaMikdash itself), they nevertheless agree that the Menorah may be built at night since its avodah (lighting it) may be performed at that time. Since in the Rambam’s view the mitzvah to build the Menorah is included in the mitzvah of building the Beis HaMikdash, part of this mitzvah is continuous and thus not considered a mitzvas assei she’hazman gramma.
The sefer, Har Hamoriah (Beis Habechirah 1:28), says that there are two parts in the mitzvah of building the Beis HaMikdash: the actual building, and the planning, measuring and bringing of supplies. Only the actual building may not be done at night. The other aspects of the mitzvah, however, may be performed at night. Hence it is not a mitzvas assei she’hazman gramma.
The Rishonim, on Kiddushin 29a, ask why the Torah feels the need to write a pasuk exempting a woman from the obligation to perform the mitzvah of bris milah on her son. After all, she should obviously be exempt since it is a mitzvas assei she’hazman gramma? The Ramban and the Ritvah answer that women are only exempt from mitzvos assei she’hazman gramma on mitzvos that pertain to themselves. But when the mitzvah requires them to do something for someone else, they are not exempt. For example, without the exemption in the pasuk, a woman would be obligated to perform a bris milah on her son.
The Minchas Chinuch (mitzvah 112:3) understands the Ritvah’s answer to mean the following: Mitzvos can be classified into two categories; those that are obligations on the individual to perform, and those that require that a certain situation take place (gavra or cheftza). The Minchas Chinuch explains that the mitzvah on the parents to perform a bris milah on their son is not a mitzvah whereby they are obligated to perform a certain act; rather that they ensure that a certain situation is accomplished – namely that their son should have a bris milah. Regarding these types of mitzvos women are not exempt, even if it is a mitzvas assei she’hazman gramma. Therefore, if the Torah did not write a pasuk that exempted women, they would be obligated to ensure that a bris milah was performed on their son.