Another interpretation: “This month shall be for you the first of months” – it is analogous to a king to whom a son was born, and he made a celebration. The son was taken captive and was there [in captivity] for a long time. After that time, the son was redeemed, and the king made a birthday celebration27The day of his redemption was celebrated like a new birthday. for him. So, until Israel descended to Egypt, they would count toward the enslavement.28God had told Abraham that his descendants would be strangers in a foreign land for four hundred years and that they would be enslaved there (see Genesis 15:13).
Once they went down and were enslaved there, and the Holy One blessed be He performed miracles on their behalf and they were redeemed, they began counting the months, as it is stated: “This month shall be for you the first of months.”