They are tied to a calendar system with its own logic, one that tracks lunar months while staying aligned to the solar year.
The Jewish calendar is lunisolar
Britannica explains that the Jewish religious year is regulated by both the Moon and the Sun. Most years contain 12 lunar months, but leap years add an extra month to keep the festivals in their proper seasons.
That is the core fact. The calendar follows lunar rhythm without letting Passover drift into winter or Sukkot into spring.
The calendar structures the whole religious year
Britannica notes that the Jewish religious year includes the cycle of Sabbaths and festivals rooted in the Hebrew Bible.
So the calendar is not merely a way to date holidays. It is one of the main ways Judaism organizes time itself.
Leap years are part of the design
Britannica states that leap years occur seven times in a 19-year cycle, with an added month before Adar.
This technical feature matters because it shows the calendar is not only symbolic. It is an applied system built to hold ritual life in season.
Why it still matters
The Jewish calendar still matters because Judaism is lived in time as much as in belief. Rest days, fasts, festivals, and mourning periods all depend on a shared temporal structure.
The shortest accurate answer
The Jewish calendar is a lunisolar system that organizes Sabbaths and festivals through lunar months adjusted to the solar year so holidays remain in their proper seasons.