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Men are obligated to put on tefillin every day. This symbolizes the absolute nature of the ties binding Israel and God. The shel yad is tied on the arm of the weaker hand, defined as the hand not used for writing (usually the left hand). The box is placed on the muscle between the elbow and the shoulder and tilted slightly toward the heart. This reminds us that all our actions (represented by the hand) and emotions (represented by the heart) should be directed to our faith. The shel rosh is placed on the head such that the entire box is above the hairline and centered. This reminds us to sanctify all our thoughts.
First, one positions the shel yad on the arm and recites the blessing: “Blessed are You, Lord our God, King of the universe, Who sanctified us with His mitzvot and commanded us to put on tefillin.” Immediately after reciting the blessing, one pulls the strap tight and wraps it around the arm seven times, finishing by wrapping it around the hand. The shel rosh is then placed on the forehead and tightened. (It is forbidden to speak between putting on the shel yad and the shel rosh.) Sephardim do not recite an additional blessing on the shel rosh. Ashkenazim recite: “Blessed are You, Lord our God, King of the universe, Who sanctified us with His mitzvot and commanded us concerning the mitzva of tefillin.”
Tefillin must be treated respectfully. While wearing them, a person may not enter a bathroom or even pass wind. He must not forget he is wearing them and must be careful not to think sinful thoughts with them on. In the past, when people often wore tefillin all day long, they had to be careful to treat them respectfully throughout the day. Additionally, since wearing tefillin clearly identified them as Jews, they had to be extra careful not to behave badly, which would have been a desecration of God’s name. At that time, it was only at night the Sages prohibited wearing tefillin, because one cannot focus properly on them or take care of them properly while asleep. Over the course of time though, people grew more concerned about their ability to treat tefillin appropriately, so the custom developed to wear them only in the morning during the Shaĥarit service (and possibly for a Torah study session immediately afterwards). The most prominent symbol identifying a person as an observant Jewish man shifted from tefillin to a kipa (skullcap, yarmulke in Yiddish).
