When you say "on the day on the Jewish calendar," does that mean that you start putting on tefilin on the same day of the Jewish calendar month in the previous Jewish month, or is it always 30 days before, which may not fall on the same day of the Jewish month? If it’s always 30 days, is it 30 days including the Bar Mitzvah day or not including the Bar Mitzvah day?
ב"ה Shalom Although, there are those who put on Tefillin 30 days before the Bar mitzvah, the more prevalent minhag is to start 2-3 months before. (See Mishna Brura 37:10, Kitzur Shulchan Aruch 10:24). According to Rav Ovadia Yoseph zt"l,(יחווה דעת ב:ד) the Sefardi minhag is to start even before, if the boy knows how to behave properly with Teffilin. If your minhag is to start 30 days before, it should be a full 30 days before the actual Bar Mitzva day, even though these 30 days include Shabbat when Tefillin is not put on. Mazal tov upon the upcoming Bar mitzvah