Aliyah to the Torah:

It is an old custom for the Bar Mitzvah boy to receive an Aliyah to the Torah on the occasion of his Bar Mitzvah^[1], in order to publicize that he has now reached the age of Mitzvos.^[2]

When:[3] The Chabad custom, received as a tradition from the Baal Shem Tov, Maggid, and Alter Rebbe, is for the Bar Mitzvah boy to receive his first Aliyah to the Torah either on a Monday, Thursday, or by Mincha of Shabbos. [Accordingly, if his birthday is on Sunday or Monday then his receives his Aliyah on Monday. If it is on Tuesday, Wednesday, or Thursday, then he receives it on Thursday. If it is on Friday or Shabbos, then he receives it by Mincha of Shabbos. Likewise, if his Bar Mitzvah is on, or in the closest proximation to, Yom Tov, Chol Hamoed, Rosh Chodesh, or a fast day, then he is to receive an Aliyah on that day.[4]] In the event that the Aliyah was received on a Monday or Thursday, he is to also receive an Aliyah on the following Shabbos by Mincha.[5]

How the boy is called to the Torah: It is customary that on the first occasion that the boy is called to the Torah for an Aliyah that he is called up as “Yaamod Hachasan HaBar Mitzvah Peloni Ben Peloni Chazzak”

The Shabbos after the Bar Mitzvah-Shacharis:[6] A boy with a Bar Mitzvah on that Shabbos is considered a Chiyuv. [Likewise, if his Bar Mitzvah fell during the week then he is considered a Chiyuv on the Shabbos after the Bar Mitzvah.[7] This, however, only applies if he did not yet get an Aliyah that week. The Chabad custom is for the Bar Mitzvah boy to receive an Aliyah by Mincha on the Shabbos following his Bar Mitzvah.[8] The father of the Bar Mitzvah boy is not considered a Chiyuv, although the custom is to give him an Aliyah.[9]]

Throwing candies:[10] Some are accustomed to throw bags of candies at the Bar Mitzvah boy at the conclusion of is Aliyah, although some negate this practice.

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[1] See M”A 225:4 “When he reads from the Torah the first Shabbos”; 282 in end that he is a Chiyuv that Shabbos; Biur Halacha 136:1 “Beshabbos Veyom Tov”;  See Piskeiy Teshuvos 136:7

[2] See M”A 225:4 regarding him reading from the Torah in public, that this would publicize the fact that he is Bar Mitzvah [indeed in previous times when it was customary to call children to the Torah, especially for Maftir, only thefact that he read from the Torah publicized the fact that he ius of Bar Mitzvah age, however, see Shaar Hatziyon 225:7 that today that we no longer call up children to the Torah, the calling up itself serves as a sign that he is Bar Mitzvah]; Levushei Mordechai Kama 37; Shaar Hatziyon 225:7

[3] Sefer Haminhagim p. 74; Toras Menachem 17:189; Likkutei Dibburim 2:266-268; 353 [Hebrew] that the Mittler Rebbe received his Aliyah on Shabbos, and not on Thursday, although the Alter Rebbe stated in the name of the Maggid, in the name of the Baal Shem Tov that one should try for the Bar Mitzvah boy to receive an Aliyah On Monday, Thursday, or by Shabbos by Mincha, although the reading of Shabbos by Mincha is the highest of them all

[4] See Igros Kodesh 17:87 ibid that one should not push it off, and that rather he should get an Aliyah by the first opportunity and then go up on Mincha of Shabbos, as explained next.

[5] Igros Kodesh 17:87 that the Bar Mitzvah boy should also go up on Shabbos by Mincha; Heichal Menachem 1:211; Shulchan menachem 3:339; See Likkutei Dibburim p. 354 that the Mittler Rebbe received his Aliyah by Mincha of Shabbos and not on Thursday

[6] M”A 282 in end; Biur Halacha 136:1 “Beshabbos Veyom Tov”;  See Piskeiy Teshuvos 136:7

[7] Biur Halacha ibid; Shvus Yaakov 2:129; See Piskeiy Teshuvos ibid

[8] Igros Kodesh 17:87

[9] Piskeiy Teshuvos 136:11

[10] Nitei Gavriel p. 74