At the end of the 12th month (February or March), which is called “Adar”, the barley crop is examined to see if it has reached the stage called “aviv”, because it must be ready to be used as the First Fruits Offering on (mostly) the 1st Sunday after Passover, which always is on the 14th of Aviv. If Passover is on the Sabbath, the offering is pushed forward an extra week, because Unleavened Bread and First Fruits feasts cannot both start on the same day. But if the barley is not yet ready, a second 12th month, called “Adar Bet” is observed. The Hebrew year is only 354 ¼ days. So, about every three years, a second twelfth month is observed.