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Which month is added during a Jewish leap year?

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Circular diagram of the Jewish religious year showing Hebrew months around the outer ring and Gregorian months inside, with major holidays and fasts such as Rosh Hashanah, Yom Kippur, Sukkot, Hanukkah, Passover, and Purim labeled with their dates. — Jewish religious year The months and holidays of the Jewish religious year.

In a Jewish leap year, an additional 30-day month called First Adar (or Adar Rishon) is added to the calendar.12

This intercalary month is inserted to ensure that the lunar-based calendar remains synchronized with the solar year, thereby keeping major festivals in their proper seasons. First Adar always precedes the month of Adar, which is referred to as Second Adar (or Adar Sheni) during a leap year. This adjustment occurs seven times during every 19-year Metonic cycle.12