The Jewish calendar is based on a lunisolar calendar, meaning it follows both the cycles of the moon and sun to track time. Jews follow the cycle of the moon to count the days in a month, or Chodesh, and the cycle of the sun to track the days of a year, or Shana.
When calculated, each month is exactly 29.5 days, 44 minutes and 2.8 seconds. Since it’s not practical to start a month in the middle of the day, months are either 29 or 30 days long.
In biblical times, a new month was determined to begin at the onset of a new moon, or Molad. At least two witnesses had to confirm seeing the new moon before the month could “officially” begin.
In order to keep the calendar on track for seasonal observances and holidays, such as Passover and Sukkot, a leap year (not day) is added seven times with a 19-year span. The leap year consists of adding another month, Adar. Watch this video from “Ask Yael” about the Jewish calendar.