The tenth plague (death of first born sons), just before Ramesses II permitted the Jews' departure from Egypt. Do you mean when was the earliest celebration of Passover? In theory, the first Passover or Pesach meal was held just before the Hebrew slaves (who later became the Jews) were released by the Pharaoh of Egypt. An exact date is not known. It is estimated that the date is about 1200 to 1500 years before the birth of Jesus or about the year 2400 in the Jewish calendar. That would be about 3,500 years ago. For more information, see * Judaism Wikia on Pesach