How the date is found
First the date of birth is turned into a Hebrew date, moving one day forward for an evening birth. The bar mitzvah is the same Hebrew date 13 years later (12 for a bat mitzvah in Orthodox practice). Like every Hebrew date, it begins at sundown the evening before.
The Torah portion shown is the one read on the first Shabbat on or after that date, following the standard yearly cycle. In the weeks after Pesach and Shavuot, Israel can be a portion ahead of communities abroad, which is why the location setting matters.
Planning ahead
Families often book two or three years ahead. Halls and synagogues fill quickly around the holidays, and many communities avoid celebrations with music during the Omer (between Pesach and Shavuot) and the Three Weeks in summer. If the Shabbat right after the birthday does not work, the child can read on a later Shabbat, or on the Monday or Thursday after the birthday.
Bar and bat mitzvah questions
At what age is a bar or bat mitzvah?
A boy becomes bar mitzvah at 13 and a girl becomes bat mitzvah at 12, counted by the Hebrew calendar. Many Reform and Conservative communities celebrate a bat mitzvah at 13; choose that option if it is your synagogue’s practice.
Which Torah portion will my child read?
Usually the portion of the first Shabbat on or after the Hebrew birthday, which the calculator shows. Some families choose a later Shabbat. Israel and the diaspora can be a portion apart for a few weeks after Pesach, so set the location.
Why is the date different from the 13th birthday on the calendar?
The Hebrew year is shorter or longer than the civil year, so the Hebrew birthday drifts. After 13 years it can fall anywhere from about three weeks earlier to two weeks later than the civil birthday.
My child was born in Adar. Which Adar counts?
In a leap year, a child born in Adar of a common year celebrates in Adar II. A child born in Adar I celebrates in Adar I if the bar mitzvah year is also a leap year, otherwise in Adar.
What if my child was born after sunset?
Choose After sunset. The Hebrew day starts at nightfall, so an evening birth belongs to the next Hebrew date. If it was close to sunset, check the local sunset time and ask your rabbi about twilight.
Sources: Shulchan Aruch, Orach Chaim 55:9–10 with Rema (age and Adar); Mishnah Avot 5:21.