Why is the letter shin embossed on the Tefillin Shel Rosh?
As an indication of the number of days in the year we put on tefillin. The Jewish lunar year has 355 days. There are 52 Shabbosos in the year, and seven days of Yom Tov according to the Torah. But two or three Shabosos a year fall on Yom Tov, so they are included in the Yomim Tovim. This is a total of 300 days, on which we wear tefillin, which is the numeric value of the letter shin. ~ Rabbeinu Bachya
Today, my son, Yaron Avraham, is IYH putting on Tefillin for the first time. My blessing to him is that he should wear his tefillin each day with pride and with simcha until 120 years. Mazel tov!