Esther 9:27-28
The Jews ordained, and took upon them, and upon their seed, and upon all such as joined themselves unto them, so as it should not fail, that they would keep these two days according to their writing, and according to their appointed time every year; And that these days should be remembered and kept throughout every generation, every family, every province, and every city; and that these days of Purim should not fail from among the Jews, nor the memorial of them perish from their seed.
Exodus 12:14
And this day shall be unto you for a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever.
The establishment of Purim as a perpetual observance follows the constitutional memorial statute pattern of Exodus 12. The Passover statute established the template for covenant memorial feasts: a specific day of deliverance designated as a permanent feast to be kept throughout every generation by ordinance forever. Mordecai and Esther's Purim ordinance replicates this statutory structure — appointed time each year, every generation, every family, every city — applying the Exodus 12 perpetual-memorial framework to the Shushan deliverance as a new covenant commemorative feast.