Why is it so important to fast on ‘Asara BeTebet?
 On the 10^th of Tebet the holy city of Jerusalem was besieged. This was the beginning of a chain of calamities that would eventually lead to the destruction of the Bet HaMiqdash. Derushé Hatam Sofer (Ribi Moshé Sofer, Derush Adar 7:4) explains that on the 10^th of Tebet it was decided in Heaven that the Bet HaMiqdash was to be destroyed. Every subsequent year a judgment is held in the heavenly courts to determine whether or not the third Bet HaMiqdash is to be rebuilt. Due to this important reason, even if the fast fell on Shabat,[1] we would be required to fast (Abudraham, O”H Siman 550), as the pasuq states (Yehezqel 24:2) “be’esem hayom hazé” indicating that the fast must be conducted on the exact day it falls on. Even on Tish’a BeAb we do not fast if it lands on Shabat. Therefore, when fasting on ‘Asara BeTebet one should realize that we are all being judged on whether or not the third Bet HaMiqdash will be rebuilt this year and we should make this fast very dear to us.
 
This year there is the rare occurrence where the fast of ‘Asara BeTebet falls out on a Friday. There are some differences when this situation occurs compared to when the fast falls on a regular weekday, see below for those differences.
 
Tora Reading
 When this fast occurs on ‘Ereb Shabat as opposed to a regular weekday, Moroccan Jews follow the Bet Yosef’s ruling (O”H Siman 550)[2] to not read the parasha of VaYehal Moshé that is read at Minha on normal fast days.[3] Some write that this is still the minhag of Meknes.[4]
 
Nevertheless, for reasons unknown it appears that the prevalent custom nowadays is to read the Tora at Minha even in Moroccan synagogues.[5]

During the Tora reading on fast days, the minhag is that when the reader reaches the Thirteen Attributes of Mercy (Shelosh ‘Esré Midot, beginning from “Ado-nai, Ado-nai”), he stops while the congregation says them aloud; he then reads them aloud himself and continues with the rest of the reading.[6] It should be mentioned that this is only done on fast days and not on Shabat Perashat Ki Tisa when the same portion is read. Qadish is not said after this reading, rather the Tora is returned to the hekhal followed by Qadish and the ‘Amida.[7]
 
Tefilin
The Bet Yosef (Siman 46) rules that tefilin must be worn during Minha on fast days since we are lacking in the 100 berakhot that one is required to say everyday due to the fact that we are not eating and miss out on those berakhot. Therefore, the minhag in Morocco developed to wear the talet and tefilin at Minha with a berakha, on all fast days, in order to arrive at these 100 berakhot.[8]

However, when a fast falls on ‘Ereb Shabat (Friday) tefilin are not worn after hasot, therefore they are not worn at Minha, even according to Moroccans.[9]

Mekorot – Sources

[1] Before our modern day set calendar, the holidays and fast days would be determined based upon when Rosh Hodesh was established. Rosh Hodesh would only be established once two witnesses came to the Bet Din and testified that they had seen the moon. Nowadays, with our fixed calendar, the 10^th of Tebet can never fall on Shabat.
 

[2] Opposing the opinion of the Agur who quotes the Shibolé HaLeqet. HaMalakh Refael Berdugo (Torot Emet, O”H 550:3) writes that the custom of Moroccan Jewry is to follow the Agur. See also Shufré DeYosef Berdugo and Ribi David ‘Obadia (Nahagu Ha’Am) who quotes the Torot Emet.
 

[3] This is because the holiness of Shabat already starts from hasot of Friday. Therefore, we do not read such perashiot. See HaMalakh Refael Berdugo (ibid.) who writes this but clarifies that on fast days that fall during the week the Tora is read at Minha, Nahagu Ha’Am (Ta’aniot), and Ribi Yosef Berdugo (Shufré DeYosef, Hilkhot Ta’anit).
 

[4] See Maghen Abot (notes of e”H Ribi Abraham ‘Amar s”t (Luah Be-ito) in the name of Ribi Dabiliski) who holds that this custom should be upheld by those who have such a tradition. E”H Ribi Yishaq Rasabi s”t writes that certain Yemenite communities did not read this Tora portion either (see S”A HaMequsar).
 

[5] Although this custom is mentioned in the above sources, it appears that it was not prevalent in most communities. In the Qisur Shulhan ‘Arukh of Ribi Refael Barukh Toledano this minhag is not mentioned. Furthermore, when the Shufré DeYosef and Nahagu Ha’Am do mention it, they do not write that this is what was practiced, they simply quote what they found in the Torot Emet. After discussing the matter with e”H Ribi Shelomo Dayan s”t (author of ‘Ateret Shelomo), and others, it was unanimously agreed that the minhag to not read the parasha is not the prevalent one, and one should indeed read from the Sefer Tora at Minha when the fast of ‘Asara BeTebet falls on a Friday. E”H Ribi David Banon s”t mentions that he was in Morocco 14 years ago when the fast fell on ‘Ereb Shabat, and e”H Ribi Shim’on Suissa s”t was poseq to read from the Tora and further mentioned that if the tefila would be at Minha Gedola, tefilin would also be worn. E”H Ribi David Peres s”t mentions that his community in Caracas, Venezuela also reads from the Sefer Tora at Minha as well as e”H Ribi ‘Amram Assayag s”t (Chief Sefaradi Rabbi of Toronto). See also Osar HaMikhtabim (Heleq 3, §829), VaYomer Yishaq (Ta’aniot Siman 2), Ma’asé Bereshit (Ta’anit), and Shoshanim LeDavid Sabah (Heleq 2, Siman 4).
 

[6] E”H Ribi Mordekhai Lebhar s”t explains that this is done in order to awaken the congregation during fast days to change their ways and start emulating the midot of their creator, Hashem, Yitbarakh Shemo La’ad. See Maran HaHida in LeDavid Emet (Siman 10:10) who says this is the minhag, Ribi Yishaq ibn Danan (LeYishaq Reah §100), Ribi Yosef Benaim (Noheg BeHokhma, Sefer Tora §4 p.141) who brings this minhag in the name of the Sefer HaHasidim (Siman 200), and Ribi David ‘Obadia (Nahagu Ha’Am, Ta’aniot, p.116).
 

[7] See the Moroccan mahzor – Hamesh Ta’aniot.
 

[8] See Ribi Yishaq Benoualid (VaYomer Yishaq, Liqutim: Ta’anit §2). So too is the custom in Fes (heard from e”H Ribi Aharon Monsonego s”t), Meknes, Marrakech, Tanger, Tetouan (e”H Ribi Ya’aqob Benaim s”t, Maghen Abot, O”H p.428 §79), and most other cities. 
 

[9] Ribi Yishaq Benoualid (VaYomer Yishaq, Heleq 1, Liqutim O”H Ta’aniot), however, rules that when the fast falls on ‘Ereb Shabat, tefilin are NOT worn because the holiness of Shabat has already presided. He bases his ruling on reasons rooted in Qabala. The posqim who mention this minhag seem to imply that even if Minha Gedola is prayed, one still does not wear tefilin because it is after hasot; see Maran HaHida in Mahziq Berakha (Siman 267) and Kaf HaHayim (Siman 25:100).

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Part 2

To clarify yesterday's halakha regarding reading the Tora on 'Asara BeTebet that falls on Friday, if Minha Gedola is prayed, the Tora is read and talet and tefilin are worn. If Minha Qetana is prayed then these three things are not done.

In Morocco there was never a question regarding reading the Tora on the 10^th of Tebet that fell on a Friday because in any case Minha Gedola was prayed in order not to be pressed for time and not to diminish the preparations for Shabat, and at such an early time for sure the Tora was read. Hence, these concerns are applicable only to Minha Qetana. Ribi David Banon of Montreal asked Ribi Shim’on Suissa, former Chief Rabbi of Morocco, about this minhag and he answered that the custom is to wear the talet and tefilin and to read Parashat VaYehal Moshé in the event that one prays Minha Gedola. However, if one prays Minha Qetana which was rare as mentioned above, then one would pray without talet and tefilin and would not read Parashat VaYehal Moshé but without question 'Anenu was said in the 'Amida. Ribi Suissa added that he clarified this with Ribi Shalom Messas who told him that this was indeed the custom everywhere besides from Meknes where the talet and tefilin were never worn at Minha of a public fast day (aside from Tish'a BeAb where tefilin are only worn at Minha and not Shaharit). Ribi David Banon writes that he himself merited to pray many years with Ribi Shalom Messas who was from Meknes, and despite that he would wear a talet and tefilin because that was not the minhag of the place he was praying in. Ribi Suissa continues that when 'Asara BeTebet fell on Friday, an effort was made to pray Minha Gedola in order to wear the talet and tefilin and read VaYehal Moshé. This was also confirmed by the Chief Rabbi of Quebec, Ribi David Sabah.

Reciting ‘Anenu
Aside from Shaharit and Minha, during ‘Arbit before the Fasts of Gedalia, Ester, ‘Asara BeTebet, and Shib’a ‘Asar BeTamuz, the Moroccan minhag is to say ‘Anenu in the ‘Amida even though the fast does not begin until the following morning.[1]
 
Birkat Kohanim
At Minha of fast days, Birkat Kohanim is performed unlike during the regular weekday Minha when it is not done.[2] However, the Shulhan ‘Arukh[3] rules to only do Birkat Kohanim on fast days when Minha Qetana is prayed (later in the afternoon).[4] Thus, neither Birkat Kohanim nor the pesuqim starting with “Eloh-enu ve-elo-hé abotenu…” (if no Kohen is present) is recited on fast days when praying Minha Gedola (before Pelag HaMinha).[5] Furthermore, as mentioned above, the minhag of Morocco is to pray Minha Gedola on fast days.[6]

Therefore, when a fast comes out on ‘Ereb Shabat (Friday) and one recites Minha as normally done right before Shabat starts (Minha Qetana), then the congregation does Birkat Kohanim.

Mekorot – Sources

[1] This is in accordance with the ruling of the Shulhan ‘Arukh (Siman 565). See Tur (Orah Hayim 565:3) and Rosh who notes that this was the custom in Spain. See also Ribi Yosef Messas (Osar HaMikhtabim Heleq 2, §38) and Ribi David ‘Obadia (Nahagu Ha’Am, Ta’aniot §24). E”H Ribi Shelomo ibn Danan of Fes (Maghen Abot, O”H p.431 §22) attests to this being the minhag as well as e”H Ribi David Sabah s”t of Marrakech (Maghen Abot, O”H p.433 §35) and e”H Ribi Ya’aqob Benaim s”t of Tetouan (Maghen Abot, O”H p.427 §75).
 

[2] The Gemara (Ta’anit 26b) cites Ribi Yosi who rules that Birkat Kohanim is done during Ne’ila of Yom Kipur but not during Minha (of Kipur). Birkat Kohanim is not done during the weekday Minha for fear that after lunch the Kohen may still be intoxicated from alcohol drunk during lunch. Since one cannot perform Birkat Kohanim in a state of stupor, the Rabbis decreed that Birkat Kohanim never be done during the weekday Minha. However, in the case of Yom Kipur, even though such a situation would not arise because the Kohen will be fasting and not consuming any alcohol, Ribi Yosi nonetheless rules that Birkat Kohanim is not performed during Minha of Kipur for fear that some may confuse the law and inadvertently do Birkat Kohanim during Minha of the week. However at Ne’ila, since it is only once a year, the Rabbis were not concerned that people would be confused. Following the same course of logic, Maran (O”H, Siman 129:1) permits performing Birkat Kohanim during Minha of regular fast days only if Minha is recited close to sunset (Minha Qetana). The reason being that this is equated to a Ne’ila service and thus people will not err. On fast days, since one does not eat we are not worried that the Kohen will become intoxicated and therefore we take this opportunity to perform it.
 

[3] O”H Siman 129
 

[4] Ribi David ‘Obadia (Sfrou, Nahagu Ha’Am, Somot VeTa’aniot §2) and e”H Ribi Yehoshu’a Maman s”t in ‘Emeq Yehoshu’a (Meknes, as quoted by Maghen Abot, O”H p.423 §2) write that the Jews of North Africa never recited Birkat Kohanim at Minha of regular fast days primarily because Minha was prayed in the afternoon close to midday (Minha Gedola). They stipulate though, that even if a synagogue prays Minha Qetana, the opinion of Maran is still not followed and Birkat Kohanim nor Elo-henu is recited in order not to differentiate and complicate matters. HaRab Yafé LaLeb of Izmir, Turkey (Siman 127:5) also writes that even when praying Minha Qetana of fast days, Birkat Kohanim is not recited.
 

[5] This is according to the Shulhan ‘Arukh in Siman 566:5. See also Nahagu Ha’Am (p.111, Somot VeTa’aniot §6), Qiriat Hana David (O”H Siman 87), Yafé LaLeb (Siman 127:5, who confirms that this was the minhag in Izmir, Turkey), and Kaf HaHayim 566:51. This is also  the ruling of e”H Ribi ‘Amram Assayag s”t and other posqim. It should be noted that Ribi Yosef Hayim (the Ben Ish Hai) rules in sefer Rab Pe’alim (Heleq 4, Siman 6) that if a Kohen does go up to do Birkat Kohanim at Minha Gedola, the congregation should not stop him. Ribi David ‘Obadia in his sefer Nahagu Ha’Am (Ta’aniot) rules that in order not to make any difference between the two Minhas, we do not recite Birkat Kohanim regardless of when Minha is recited. E”H Ribi Yehoshu’a Maman s”t in ‘Emeq Yehoshu’a as quoted by Maghen Abot (O”H p.423 §2) cites the minhag of Meknes being that generally Minha Gedola would be prayed and thus Birkat Kohanim is not done, but adds that even if Minha Qetana was prayed it is still not done.
 

[6] See Nahagu Ha’Am (ibid.), Qiriat Hana David (Siman 87), Shoshanim LeDavid Sabah (Heleq 2 Siman 4), and ‘Emeq Yehoshu’a (as quoted by Maghen Abot, O”H p.423 §2).