Sometimes my tefillin shel yad slips out of place and needs refastening. Should I take off my shel rosh and put the two on in the right order?
You assume correctly that the order of putting on the shel yad before the shel rosh is fundamental. The gemara (Menachot 36a) derives this from the order of the p’sukim, which all mention tying the tefillin to the arm before having tefillin on the head. In fact, despite the concept that one should not “pass over” one mitzva to get to another, if one first picked up a shel rosh, he should put it down in favor of the shel yad (Shulchan Aruch, Orach Chayim 25:6). The gemara derives further that we take off the shel rosh before first from, “v’hayu l’totafot bein einecha” (they shall be tefillin shel rosh between your eyes). The plural language indicates that when the tefillin are on your head, you should have two tefillin on.
Is it actually some sort of transgression to have the shel rosh on without the shel yad, or are the aforementioned just instructions of the optimal order. There certainly cannot be an absolute transgression because the mishna (ibid. 38a) says that the absence of one of the tefillin does not preclude putting on the other. If, as it sounds, the proper fulfillment of the mitzva is when the shel yad is put on before the shel rosh but either one still has value, one can understand the Taz’s (OC 684:4) following claim. There is a preference to put on the shel yad first and the mitzva’s full fulfillment comes only when both tefillin are on. If one puts on the shel rosh first it has value, but one should get to the state of having both on at the first opportunity. One accomplishes this, not by removing the shel rosh, which delays matters, but by putting on the shel yad immediately. The Taz’s opinion is accepted as halacha (Pri Megadim, EA 25:7; R. Akiva Eiger to 25:6; Mishna Berura 25:22).
Not all agree. The Avudraham (cited by the Taz, ibid.) compares the case of one who puts the shel rosh on before the shel yad to a community that reads the Chanuka Torah reading before that of Rosh Chodesh when the two overlap. He cites a source that regarding the tefillin one does not fulfill the mitzva because of the mistaken order and posits that the same is true for the Torah reading. Regarding Chanuka, the Rama (684:3) accepts the Avudraham’s opinion. The Taz takes issue with the Avudraham, firstly because the apparently Talmudic source he cites is not found in our texts. It is possible also to accept the Avudraham/ Rama’s opinion regarding Chanuka based on local considerations and not regarding tefillin (see the Biur Halacha to 25:6 in the Gra’s name).
Only the Malbim (Artzot Hachayim 25:43) accepts the Avudraham regarding tefillin. He cites statements of Chazal that indicate that putting on the shel rosh first is a sin, and thus if one put on the shel rosh first, that should be disregarded. It appears (and the Avudraham likely agrees) that if one put on the tefillin in the right order and the shel yad later slipped that all would agree that one returns the shel yad to its proper position without removing the shel rosh.
It is noteworthy that many Acharonim do not follow the Shulchan Aruch’s (OC 25:12) ruling to make a beracha when returning tefillin that slipped out of place. The Shelah explains that, when making the original beracha, one’s intention is to have it extend until the end of tefilla regardless of such breaks. Although there is not unanimity on the matter, that seems to be the more accepted practice (Mishna Berura 25:42; Kaf Hachayim 25:77). Therefore, it is not necessary to determine if the tefillin slipped so much to break the fulfillment of the mitzva. This along with the answer to your question allows one to rectify the situation and continue tefilla without fanfare, which is generally best.
[Machon Eretz Chemda]
See a beautiful tshuva in the Yabia Omer [10/2].
