When in the morning may one begin to put on Tefillin:
Due to the prohibition against wearing Tefillin at night lest one fall asleep in them, it is therefore forbidden to put Tefillin on in the morning until the time that most people awaken. This is defined as the time of morning in which it is light enough to recognize an acquaintance from a distance of 4 cubits. This time period is formally known as “Mi-Sheyakir”. It is forbidden to wear the Tefillin prior to this time, even though it is already after Alos Hashachar. 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 one’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 is traveling on foot in the early morning, prior to daybreak, is allowed to don Tefillin without a blessing even prior to the above mentioned time of Mi Sheyakir. When the time of Mi Sheyakir arrives, he is to say move the Tefillin and then say the blessing. If he is not traveling by foot, but rather through some form of transportation, such as 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 Mishayakir?
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 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?
Some Poskim rule that if one knows he will not be able to put on Tefillin for even one moment that day, then he may place them on upon awakening, even if it is prior to Alos. However one may not recite a blessing on the Tefillin, although some Poskim are lenient.
 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.
 The reason: As there is no worry that he may fall asleep while walking. [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 Seafradim] 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]
 Igros Moshe 1/10; Piskeiy Teshuvos 30/1 in name of Poskim
 The reason: As one has already awoken and there is thus no worry that he may fall asleep. [Igros Moshe ibid]
 Admur ibid regarding a traveler; Piskeiy Teshuvos ibid
 Igros Moshe ibid