When in the morning may one begin to put on Tefillin:
It is forbidden to put on Tefillin [at night and] even after Alos Hashachar, until the time that majority of people awaken from their sleep. This is defined as the time of morning in which it is light enough to recognize a person which one is slightly acquainted with, from a distance of four cubits. [This time period is formally known as â€œMi-Sheyakirâ€. The exact time of Mi-Sheyakir is disputed and fluctuates based on city and time of year. See Q&A below for the full details on this subject! Practically, to verify the exact time of Mi-Sheyakir in oneâ€™s city for that day, one is to look at a Jewish calendar/Luach that lists his location and his custom.]
A traveler: One who desires to travel on foot in the early morning, prior to daybreak, may put on Tefillin without a blessing [even though it is prior to the above mentioned time of Mi Sheyakir]. When the time of Mi Sheyakir arrives, he is to move the Tefillin and then say the blessing. If he is not traveling by foot but rather through some form of transportation, such as one who is riding [on a horse] or sitting in a car, train or plane, it is forbidden to wear the Tefillin before Mi Sheyakir, even without a blessing.
One may begin to don Tefillin with a blessing beginning from Mi-Sheyakir, when there is enough light after dawn to be able to recognize an acquaintance from four Amos. It is forbidden to don Tefillin before this time, even if it is past Alos, unless one is traveling by foot prior to Mi Sheyakir.
Practically, when is the exact time of Mi-shayakir?
The exact time of Mi-Sheyakir is difficult to ascertain, and there are various opinions in its regard. Practically, the widespread custom in Jerusalem/Eretz Yisrael is to consider Mi Sheyakir exactly 60 minutes before sunrise throughout all times of the year, although other customs do exist. The widespread custom in the Diaspora is to consider Mi Sheyakir either from when the sun is 12.9 Degrees from the horizon, or 11.5 degrees from the horizon. Practically, each person is to follow a calendar that contains the listing of his city, and follows the Halachic opinion that he is accustomed to follow. There is no known Chabad custom in this regard.
If one must begin Davening before Mi Sheyakir, when is he to put on the Tefillin?
He is to begin Davening without Tefillin, and then place the Tefillin on after Mi Sheyakir. One is to try to arrange to place the Tefillin one between Yishtabach and Yotzer Or.
If one is unsure if Mi Sheyakir has arrived, may he wear the Tefillin?
If one knows that he will not be able to put on Tefillin the entire day, even for a moment, may he put them on in the early morning, prior to Mi Sheyakir?
If one knows he will not be able to put on Tefillin for even one moment that day, as can occur by one who works throughout all the daylight hours, then he may place them on upon awakening, even if it is prior to Alos. He is permitted to recite a blessing upon wearing them at this time if he will be unable to wear them at all after Mi Sheyakir, and throughout the day. Nevertheless, it is best for a Torah scholar not to say a blessing upon wearing it.
If one is in doubt as to whether the time of Mi Sheyakir has arrived, may he wear Tefillin?
If one put on Tefillin with a blessing prior to Mi Sheyakir, is he to recite a blessing afterwards?
 30/1; Michaber 30/1
 Admur ibid; Michaber ibid; Ramban Brachos
The reason: Although starting from Alos Hashachar it is considered daytime for all the Mitzvos of the Torah, nevertheless it is forbidden to wear Tefillin until the time that majority of people awaken, justa s it is forbidden tow ear Tefillin at night, lest one fall asleep in the Tefillin. [Admur ibid]
 If one is very well acquainted with the person then it he can be recognized even from far, and if one is not acquainted with the person at all, then he wont recognize him even from close. Therefore the above time is defined by recognizing a person whom one is slightly acquainted with. [Admur ibid; Taz 30/2]
 Admur ibid; Michaber ibid; Abayey in Brachos 9b
 Kaf Hachaim 18/18
 One needs to use a calendar that shows the times of a) his geographical location and b) the accustomed opinion that his community rules accordingly regarding the time of Mi Sheyakir.
 30/3; Michaber 30/3; Menachos 36a
Other opinions: Some Poskim rule one is even allowed to say a blessing upon wearing the Tefillin at night prior to traveling. [Rabbeinu Peretz, in Hagahos Semak 153, brought in Tur 30; See Divrei Efraim Emek Habacha 2]
 The reason: As there is no worry that he may fall asleep while wearing the Tefillin, as he has already awoken and is in the midst of traveling by foot, and will not fall asleep while walking. [Admur ibid]
The reason a blessing is not recited: As perhaps the Sages did not desire to establish a blessing at night if he could anyways say it during the day. [Igros Moshe 1/10; See Divrei Efraim Emek Habacha 2]
 Admur ibid; Mâ€A 30/5; Bach 30; Mâ€B 30/14; Biur Halacha 30/3 â€œKeivanâ€ in name of Artzos Hachaim
The reason: As one who rides or sits inside the transportation vehicle can fall asleep. [Admur ibid]
Other opinions: Some Poskim rule that one who is riding on a horse may also wear the Tefillin prior to Mi Sheyakir, although if he is sitting in a car and the like is it forbidden. [Taz 30/5; Elya Raba 30/3; Chayeh Adam 14/17; Machatzis Hashekel on Mâ€A ibid; Poskim in Kaf Hachaim 30/12] Some Poskim rule that even one who travels by transporation may wear the Tefillin prior to Mi Sheyakir. [Ateres Zekeinim 30/2; Pâ€M 30 Aâ€A 5; See Kaf Hachaim ibid]
 See Piskeiy Teshuvos 18/5
 This is dependent on various factors such as that 1) It must be a clear day without clouds and without snow, and without a bright moon or other street lights. 2) It is to be measured in a flat plane or on a mountain and not in a valley. [See Ashel Avraham Butchach 58; Nimukei Orach Chaim 58; Toras Chaim Sofer 58/2; Igros Moshe 4/6] 3) We no longer have Techeiles today to compare the white to. [See Aruch Hashulchan 18/9; Birchas Habayis 37/37; Shulchan Shlomo 18/2]
 Some hold it is exactly 60 minutes before sunrise. [Kaf Hachaim 18/18 as Minhag Yerushalayim; Sefer Eretz Yisrael of Rav Tukichinsky that so is custom of Sephardim] Others hold it is 52 minutes before sunrise. [Nivreshes p. 43 as custom of Eretz Yisrael] Others say it is 57 minutes before sunrise. Others say it is 42 minutes before sunrise. [See Birur Halacha 18 and Beis Baruch on Chayeh Adam 11] Others say it is 35-40 minutes before sunrise. [Igros Moshe 4/6] Others are lenient to consider Mi Sheyakir from 6 minutes after Alos. [Peri Megadim; Rav Ovadia Yosef]
 Kaf Hachaim ibid
 See previous footnotes!
 12.9 degrees corresponds to the opinion of 60 minutes before sunrise in Jerusalem. 11.5 degrees corresponds to the opinion of 52 minutes from sunrise in Jerusalem. Some calendars measure from 11 degrees or 10.2 degrees from the horizon. [My Zmanim in opinion of Kovetz Otzros Yerushalayim]
 Piskeiy Teshuvos 30/1
 Shoel Umeishiv Gimel 2/162; Piskeiy Teshuvos 30/1
 Yosef Ometz 219; Mishneh Sachir 7
 Igros Moshe 1/10; Piskeiy Teshuvos 30/1 in name of Poskim; Divrei Efraim in Emek Habracha 2 that so he ruled in times of the Holocaust in the ghettos and camps; Heard from Rabbi Mingal that when he was nine years of age in Aushwitz all the men lined up in middle of the night and put on his fatherâ€™s Tefillin with a blessing.
 The reason: As we rule that Biblcially the Mitzvah of Tefillin applies even at night and it is only due to worry of falling asleep that we decree against wearing Tefillin at night. Hence, if one already woke up from his sleep and will not be able to put on the Tefillin at any other time, then one may wear it then as there is no worry that he will fall asleep. [Igros Moshe ibid; Divrei Efraim ibid] We have no proof to say that the Sages decreed against wearing Tefillin at night even if this will uproot the Mitzvah completely. [Divrei Efraim ibid]
 Poskim ibid
The reason: As Rabbeinu Peretz rules that a blesisng may be said at night by a traveler, and although we do not rule like his opinion, nevertheless perhaps in this case that he will not be able to say a blessing later on all would agree that he can say a blessing at this time. [Igros Moshe ibid]
 Igros Moshe ibid
 Shoel Umeishiv Mahadurah 3 2/162; Kaf Hachaim 18/19
 Elya Raba 30/3; Birkeiy Yosef 30/3 and Kesher Gudal 3/19; Makor Chaim 30; Shalmei Tzibur p. 41; Chesed Lealafim 30/2; Rav Akiva Eiger 30/13 in name of Rabbeinu Peretz; Shaareiy Teshuvah 30/7; Birchas Habayis 38/19; Ben Ish Chaiy Chayeh Sara 8; Mâ€B 30/13; Biur Halacha 30/3 â€œUkesheyagiaâ€; Kaf Hachaim 30/11 in name of Poskim
Other opinions: Some Poskim rule that the blesisng must repeated after Mi Sheyakir if it was said beforehand. [Os Chaim Veshalom 30/1 based on Maharil brought in Shaareiy Teshuvah 664]