1. The start of night-Until when during the day may one put on Tefillin?[1]

One is not to put on Tefillin after the start of sunset [i.e. Bein Hashmashos].[2] [Hence one is to be very careful to always put on both the Tefillin of Rashi and Rabbeinu Tam prior to sunset, if for whatever reason he did not do so in the morning prior to Shacharis. Accordingly, on Tishe Beav when the Tefillin is only worn by the time of Mincha, one must be careful to finish Mincha with enough time before sunset to put on Tefillin of Rabbeinu Tam prior to sunset. If the Minyan is delayed, then one should put on Rabbeinu Tam even prior to the conclusion of Mincha.]

[1] Siddur Admur [letter 19 in Siddur of Raskin]; Implication of Beis Yosef 30; Opinion in P”M 30 A”A 2; M”B 30:3 and Biur Halacha 30:5 “Veyeish” in name of P”M that he questions the ruling of the M”A; See Rambam Tefillin 4:10-11 who writes the Biblical prohibition of wearing Tefillin begins from sunset. [However, a) perhaps it is Lav Davka, and the intent of sunset is nightfall and b) perhaps this only applies in his opinion that the prohibition to wear Tefillin at night is Biblical]

Ruling of Kabalah: According to Kabbalah, one is to remove his Tefillin prior to sunset, and certainly is not to initially wear it after sunset, and so was the custom of the Arizal. [Yad Ahron in Hagahos Tur; Birkeiy Yosef 30:2; Shalmei Tzibur p. 9; Siddur Beis Oved; Shaareiy Teshuvah 30:3; Yifei Laleiv 30:2; Kaf Hachaim 18:11-12; 30:5 and 7]

Ruling of Admur in Shulchan Aruch and other Poskim: Some Poskim  rule that one may don Tefillin until nightfall/Tzeis Hakochavim, and so is the ruling of Admur in his Shulchan Aruch. [Admur 30:2 “If he put them on before Tzeis Hakochavim” [see also Admur 18:8 regarding Tzitzis]; M”A 30:2; Mordechai [brought in Beis Yosef 30]; M”B 30:3 that from the M”A it is implied that it may even initially be worn during Bein Hashmashos and Biur Halacha 30:5 “Veyeish” that “certainly he is obligated to wear them during Bein Hashamshos if he has yet to wear the Tefillin, as Sfeika Lechumra and perhaps it is still day”] Some Poskim rule one may wear it even with a blessing until Tzeis Hakochavim. [Chayeh Adam 14:16; Artzos Hachaim 30; Shulchan Shlomo 30; Piskeiy Teshuvos 30:2 footnote 11-12] Other Poskim rule one is to wear it without a blessing. [Kaf Hachaim 30:17; Biur Halacha 30:5 “Veyeish” concludes to wear the Tefillin without a blessing] After nightfall, according to all opinions it is forbidden to wear Tefillin, as stated above.

[2] The reason: Perhaps Admur in the Siddur follows the ruling of the Rambam who Biblically prohibits wearing Tefillin after sunset. [See Rambam 4:10; footnote 78 in Siddur Raskin] Alternatively, Admur in the Siddur suspects for the opinion of the Arizal who prohibits wearing Tefillin at night. [Piskeiy Teshuvos ibid]

[3] P”M 30 A”A 6; M”B 30:3 and Biur Halacha 30:5 “Veyeish” that “certainly he is obligated to wear them during Bein Hashamshos if he has yet to wear the Tefillin, as Sfeika Lechumra and perhaps it is still day”; Kaf Hachaim 30:17 that this applies even according to the Arizal; Rav Ginzberg in Hiskashrus 889; Rav Rubinovitch in Hiskashrus 807, this is in contrast to the ruling suggested by Rav Raskin that after sunset it is forbidden.

Does this apply even according to Admur in the Siddur? Some write that the ruling of Admur in the Siddur does not apply to one who did not yet don Tefillin that day. [Rav Ginzberg in Hiskashrus 889; Author of Piskeiy Teshuvos brought in Hiskashrus 807; See Kaf Hachaim ibid] Others, however, argue that perhaps according to Admur in the Siddur doing so is forbidden. [Rav Raskin in Miluim on Siddur]

[4] M”B ibid; Kaf Hachaim ibid

The reason: As perhaps it is already nighttime, and Safek Brachos Lihakel. [Poskim ibid]

Other opinions: See previous footnotes.

[5] Poskim of previous Q&A; This applies even in accordance to those who follow the ruling of Admur in the Siddur: Rav Ginzberg in Hiskashrus 783:787:800:807; Author of Piskeiy Teshuvos brought in Hiskashrus 807

[6] Author of Piskeiy Teshuvos ibid

[7] As many Poskim rule Tefillin may be worn even initially until nightfall with a blessing, and so rules Admur in 30:2. Hence, why be stringent on them like the ruling of the Siddur. [ibid]

[8] The reason: As today we are accustomed to consider Tzeis Hakochavim of the Geonim as Tzeis Hakochavim for all matters. However, see Piskeiy Teshuvos 30:2 footnote 10

[9] See Piskeiy Teshuvos 30:2; Yisrael Vehazmanim [Horpanez] 3 [is lenient to permit wearing until Tzeis of Rabbeinu Tam even Tefillin of Rashi although negates initially doing so even with Tefillin of Rabbeinu Tam, and for us that we hold like Geonim, this would apply even Bedieved]

[10] The reason: As the two disputes are completely independent of each other, and we hold that after Tzeis Hakochavim of the Geonim it is nighttime. It is forbidden to wear tefillin at night, and we are initially stringent to wear them before sunset, and only in the case of Bedieved do we allow putting them on until nightfall. In this regard one must follow the nightfall of the Geonim, which is the accepted ruling today both for leniency and stringency. The fact that Rabbeinu Tam is of the opinion that nightfall is later does not affect in any way the law regarding wearing Tefillin of Rabbeinu Tam. The dispute regarding tefillin of Rashi versus Rabbeinu Tam and the dispute regarding the time of night of the Geonim versus Rabbeinu Tam are two completely different disputes which have nothing to do with each other, and while regarding the dispute of Rashi versus Rabbeinu Tam tefillin we are stringent to also put on Rabbeinu Tam, regarding the dispute of the time of night we are completely lenient like the Geonim. Accordingly, there is no room to be lenient to put on tefillin of Rabbeinu Tam past nighttime of the Geonim [which is the regular time of night listed in all calendars], and transgress the prohibition against wearing tefillin at night. To what is this similar? The same way that nobody would argue to say that the Shabbos prohibitions which follow the opinion of Rabbeinu Tam may be transgressed until the time of Rabbeinu Tam arrives on Erev Shabbos [i.e. setting up a Mechitza Hamateres on Shabbos; Squeezing inedible fruits; immersing on Shabbos if husband not in city; ], so too here the two should not be connected.