What is the Feast of Purim?

Esther 9:26-28 KJV Wherefore they called these days Purim after the name of Pur. Therefore for all the words of this letter, and of that which they had seen concerning this matter, and which had come unto them, [27] 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; [28] 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.