Memorial is supposed to be held on Aviv(Nissan ) 14 . The following shows how it is determined .
The Biblical year begins with the first New Moon after the barely in Isreal reaches the stage in its ripeness called Abib (Aviv) The period between one year and the next is either 12 or 13 lunar months . Because of this , it is important to check the state of the Barley crops at the end of the 12th month . If the Barley is Abib at this time , then the following New Moon is Hodesh Ha-Aviv ( New Moon of the Abib ) . If the barley is still immature , we must wait another month and then check the barley again at the end of the 13th month .
Ex13:4 Deut. 16:1
So it is impossible to determine the date yrs in advance . According to the scriptures . It was changed be 359 C.E , just before the Sanhedrin disbanding themselves , the exiled Sanhedrin devised a new calander system . In the 4th century Sanhedrin , Hillel 2 borrowd Babylonian mathmatical calculations to determine the precise moment that the moon was in conjuctions with the earth and the sun . This unobservable mathematical moment in time was used to determine the beginning of each month on the new Pharisee calander . So the could celebrate their adaption of the Passover at their own convience . They could plan their religoius activities months , years in advance . They completely disregarded the ONLY criterion stipulated by the Almighty for determining the first month of the year - the aviv barley in the land of Isreal .
We follow this mistake . Memorial can not be determined a yr in advance, actually
not even a mth in advance . So the dating the organization uses is often incorrect .