Those are the events that surround Passover each year. It starts with the examination of the barley crop at the end of the 12th month, called Adar, to see if it will be ready for the first fruits offering. If it is not ripe enough, a 2nd 12th month is declared, named Adar bet. At the end of the 2nd 12th month, the first month of the year is declared, called Aviv, when the first sliver of the renewed Moon is seen, or 30 days after the start of the 2nd 12th month. When there is cloud cover and the renewed Moon can’t be seen, a 30th day is declared by default, and the next evening starts the new month.