In Jewish tradition, the Hebrew calendar determines the date when bar or bat mitzvah status begins.
Jewish religious dates follow a lunisolar calendar based on lunar months and adjusted to keep festivals in their appropriate seasons. Because of that system, a child’s Hebrew birthday can fall on a different Gregorian date from year to year.
Families commonly calculate the milestone by matching the Hebrew day and month of birth to the year in which the child reaches the traditional age. A civil-calendar party may be scheduled nearby for convenience, but the religious date remains tied to the Hebrew calendar.
This distinction explains why Jewish coming-of-age celebrations do not always occur on the same numbered date each year. It also prevents confusion when a Hebrew date falls near sunset, since Jewish days traditionally begin in the evening.