Community Service Announcement from Rav Yisroel Bollag, rosh kollel of Kollel Oholei Yom Tov of Toronto: Will Bar Mitzvah Bochurim Miss Their First Day of Tefillin? This year is one of the rare cases that the bar mitzvah of some boys and their subsequent requirement to put on Tefillin will take place before what they would normally consider to be their Hebrew Birthday. Boys who were born on the 1st of Kislev thirteen years ago, (November 27, 2011) will not become Bar Mitzvah this year on the 1st of Kislev, but rather one day earlier, on the 1st day of Rosh Chodesh Kislev, which falls out this year on the 30th of Cheshvan, (Sunday, December 1, 2024). (Mishna Berura Siman 55 S'if Katan 45). This topic is further discussed in Shaarei Teshuva ibid. and one should consult with their local Rov. People who are not aware of this may inadvertently miss their first day of Tefillin and other Mitzvos.
As reported in TGV from MNG.