1) The practice is to don tallit and tefillin only after one has recited all the morning berachot. Ideally, the tefillin should be donned before one recites the Shema in the korbanot section of the prayer service, or at least before one recites the tamid paragraph of the korbanot section.[22]

2) In principle, the obligation of tefillin requires wearing the tefillin throughout the entire day. However, since most people are incapable of conducting themselves throughout the day in the manner required while wearing tefillin — refraining from passing air, and keeping one’s mind focused on the tefillin — the custom developed not to wear tefillin the entire day. One must, however, ensure to wear tefillin during the shaharit service, as during this brief period it is indeed possible to conduct oneself in the manner required while wearing tefillin. The Sages expressed the importance of wearing tefillin during the morning prayer service when they declared, “Whoever lays tefillin, recites Shema and prays is considered as though he has wholeheartedly accepted upon himself the yoke of Divine kingship” (Berachot 15a). It is particularly important to wear tefillin during the recitation of Shema, which includes the commandment to wear tefillin. The Sages commented regarding one who recites Shema without wearing tefillin, כאילו מעיד עדות שקר בעצמו  — he is considered as though he renders false testimony about himself. Since he speaks about an obligation that he does not fulfill at that moment, he gives the impression that he has no interest in fulfilling the will of his Creator, Heaven forbid.[23]

a. It is forbidden to wear tefillin during the nighttime hours, given the possibility that one might fall asleep while wearing them. The “nighttime hours” for the purposes of this halachah extend from sundown until the point known as “mi’she’yakir.” This refers to the time in the morning when there is enough light to enable one to recognize an acquaintance from a distance of four amot (cubits). The exact time of mi’she’yakir is printed in calendars.[24 ]

b. One should preferably don tefillin before the Shema recitation in the korbanot section of the prayer service, or at least before reciting the tamid section.[25]

c. If one does not have tefillin with him and suspects that he may not have access to tefillin until after the final time for Shema, he should recite Shema without tefillin and then don tefillin later when tefillin become accessible. This applies as well to a person who has tefillin but suspects that the final time for Shema will pass by the time he puts on the tefillin; he should first recite the Shema without tefillin and then don the tefillin. Since he genuinely intends to wear tefillin later in the day, he is not considered as “rendering false testimony about himself” and he may recite Shema without tefillin. In both these cases, the individual should then repeat the Shema recitation while wearing tefillin.[26]

d. If a person does not yet have tefillin and he suspects that if he waits for the tefillin to arrive he will miss the congregational recitation of the Amidah, and no later minyan is available, he should recite the Amidah together with the congregation without tefillin. He should then don his tefillin later once they arrive, and recite Shema while wearing the tefillin.[27]

e. If a person has with him only tefillin Rabbenu Tam, but not the standard tefillin Rashi, and if he waits for his tefillin Rashi he will miss the opportunity to pray with a minyan, he should wear the tefillin Rabbenu Tam, without a berachah, and pray with the minyan. (This applies even to unmarried men, who generally do not wear tefillin Rabbenu Tam.) Later, when he receives the tefillin Rashi, he should put them on with a berachah and repeat Shema.[28]

f. If a person forgot to add ya’aleh ve’yavo during the Amidah of shaharit on Rosh Hodesh, and he realizes his mistake only after musaf, at which point his tefillin have already been removed, he should preferably don his tefillin again before he repeats the Amidah.[29]

3) If one did not have access to tefillin while reciting the morning prayers, or at the time he suffered from a stomach virus that precluded the possibility of his wearing tefillin, then he may — and must — wear tefillin sometime later during the day, before sundown. He must lay tefillin at some point during the day so that a day does not pass without his wearing tefillin. If one did not wear tefillin at all during the day until after sunset, he may still lay tefillin with a berachah until the point of tzet ha’kochavim (nightfall), unless he already recited arvit, in which case he should lay tefillin without a berachah.[30]

4) It is forbidden to wear tefillin on Shabbat, Yom Tov or Hol Ha’mo’ed. Tefillin are worn to serve as a “sign” of our covenant with God, and Shabbat and festivals themselves constitute a “sign” of the covenant. Wearing tefillin on these occasions would thus be disrespectful to their role as a “sign.”[31]

a. According to some authorities, one may not even move tefillin on Shabbat or Yom Tov, unless he intends to use the tefillin in some permissible fashion, or if he needs the space it currently occupies. Thus, for example, if a teacher wishes on Shabbat to show his students the manner in which the tefillin are produced and how the straps are tied, he may show them an actual pair of tefillin for demonstration. Likewise, if tefillin occupy space on the table and a person needs that space, he may move the tefillin. In addition, one may move tefillin if they are in a place where they may likely be stolen, damaged or soiled.[32]

b. If one who follows the custom not to wear tefillin on Hol Ha’mo’ed prays on Hol Ha’mo’ed with a congre gation of people who do wear tefillin, he should nevertheless follow his custom and not wear tefillin.[33]

c. If a resident of Israel spends Yom Tov outside Israel and intends to return to Israel, then on the second day of Yom Tov (which is observed only in the Diaspora) he should lay tefillin privately in the home where he stays. He should recite Shema while wearing tefillin, and then remove the tefillin and go to the synagogue to pray with the congregation.[34]

5) Tefillin are worn on Purim. During the reading of the Megilah, when the reader reaches the verse ליהודים היתה אורה ושמחה וששון ויקר (“The Jews experienced light, joy, jubilation and honor” — 8:16), one should touch his tefillin shel rosh and then kiss his hand.[35]