The Hebrew calendar determines the date of a Jewish child’s bar or bat mitzvah, rather than the Gregorian calendar alone.
Jewish birthdays are calculated according to the Hebrew calendar, a lunisolar system whose months follow lunar cycles while the year is kept aligned with the solar seasons. As a result, the Gregorian date of a bar or bat mitzvah changes from year to year, while the Hebrew date remains the key religious reference.
A child generally becomes bar or bat mitzvah on the relevant Hebrew birthday, and the ceremony is scheduled around the Torah portion and synagogue service closest to that date. Families therefore consult both calendars when planning invitations, travel, and celebrations.
The Hebrew calendar includes leap years with an added month, Adar I, to keep festivals in their appropriate seasons. This is why Jewish holidays and milestone ceremonies do not fall on the same Gregorian date annually. Local practice can affect the service date, especially when the Hebrew birthday coincides with Shabbat, a festival, or another special day.