Question:
May a boy receive an aliyah and read from the Torah before his actual Bar Mitzvah date, such as on the Shabbos or Monday/Thursday prior, if that timing is more convenient?
Answer:
For Ashkenazim, this should generally not be done. The custom is not to give an aliyah to a child before Bar Mitzvah, aside from the aliyah of Maftir on Shabbos, which some allow—but not in Chabad, where this is not practiced at all. For Sephardim, there is more room to be lenient. A boy may receive an aliyah before his Bar Mitzvah, whether on Shabbos or Monday/Thursday, and may even read his own aliyah. However, he should not read the majority of the parsha. An adult should read the rest of the aliyos.
Explanation:
From the letter of the law, a child who has reached the age of understanding (chinuch) may receive an aliyah on Shabbos and even be counted among the required aliyos, provided that adults receive the remaining aliyos. However, the accepted custom today in Ashkenazi communities is not to give aliyos to children below Bar Mitzvah, with limited exception.
Ashkenazi / Chabad practice:
Practically, the custom is not to give a child an aliyah before Bar Mitzvah at all. The only common exception in some communities is Maftir on Shabbos prior to the Bar Mitzvah. However, the Chabad custom is to be stringent and not to allow even Maftir before Bar Mitzvah. Additionally, a child below Bar Mitzvah may not serve as the Baal Korei (Torah reader) on behalf of the congregation. Therefore, he may not read his aliyah or any part of the Torah reading.
Sephardic practice:
Among Sephardim, there are more lenient practices. Some permit a child below Bar Mitzvah to receive an aliyah when needed, even before his Bar Mitzvah date. In such cases, he may also read his own aliyah, provided that the majority of the Torah reading is performed by an adult. He should not read the entire parsha or the majority of it for the congregation.
Maftir and Haftorah:
A child who understands the blessings may, according to the letter of the law, receive Maftir and read the Haftorah. Many communities allow this on the Shabbos prior to the Bar Mitzvah. Nevertheless, Chassidic custom, including Chabad, is to refrain from this as well.
Accordingly, while there is some basis in halacha for a child to receive an aliyah before Bar Mitzvah, the prevailing Ashkenazi (and especially Chabad) custom is to avoid it, whereas some Sephardic communities are more lenient under certain conditions.
Sources:
Admur 282:5–7, 16; 284:8–9; Michaber O.C. 282:3; 284:4; Megillah 23a, 24a; Rambam Hilchos Tefillah 12:17; Rama 282:3–4; Magen Avraham 282:5–6, 12; Mishnah Berurah 282:11–13; 282:23; Kaf Hachaim 282:22–24, 40; Piskei Teshuvos 282:5, 8; 135:22; Yechaveh Daas 2:15; 4:23; Tzitz Eliezer 7:1; Toras Menachem 5745; Igros Kodesh 17:87