Question: We were decorating our daughter’s bedroom and removed the mezuza case only to find to that it was empty. We have ordered a new mezuza. Can she sleep there without a mezuza until it arrives?
Answer: The Shulchan Aruch (OC 38:12) writes that if one cannot afford to buy oneself both tefillin and a mezuza and needs to choose one over the other, they should buy a pair of tefillin. The Rema (YD 285:1) explains that this is because tefillin is a mitzva that pertains to the individual, as opposed to mezuza which belongs to the house. However, the Magen Avraham (38:15) writes that as we no longer wear tefillin all day, one should rather buy a mezuza and borrow someone else’s tefillin. The Pri Megadim (Eshel Avraham 38:15) adds that while tefillin are considered holier than mezuzos, the mitzva of mezuza applies on Shabbos and Yom Tov. Therefore, one must move out of a room without a mezuza on Shabbos or Yom Tov if they have another room available.
The Pischei Teshuva (YD 285:1), Ben Ish Chai (Ki Savo 2:2) and Aruch Hashulchan (YD 285:5) add that this applies equally during the week. If there is no mezuza on the door, one should move to another room if there is one available.
The Magen Avraham (13:8, quoting the Mordechai) and Sedei Chemed (40:115), however, write that one does not need to leave their house without a mezuza (See Tzitz Eliezer 13:53).
R’ Avraham Dovid Wahrman (Daas Kedoshim, Mikdash Me’at 285:3) suggests that one may declare their house hefker, ownerless, thereby obviating the need to affix a mezuza. Nonetheless, the Sedei Chemed concludes that this is not necessary, and one may stay in a house without a mezuza.
In conclusion, one should try one’s utmost to affix a mezuza as soon as one can. In the meantime, it is preferable for your daughter to sleep in another room. If that is not feasible, then she can stay there without one.