The Book of Esther is a book in the Ketuvim

Ketuvim

Ketuvim or Kəṯûḇîm in actual Biblical Hebrew is the third and final section of the Tanak , after Torah and Nevi'im . In English translations of the Hebrew Bible, this section is usually entitled "Writings" or "Hagiographa"...

 ("writings"), the third section of the Jewish Tanakh

Tanakh

The Tanakh is a name used in Judaism for the canon of the Hebrew Bible. The Tanakh is also known as the Masoretic Text or the Miqra. The name is an acronym formed from the initial Hebrew letters of the Masoretic Text's three traditional subdivisions: The Torah , Nevi'im and Ketuvim —hence...

 (also called the Hebrew Bible

Hebrew Bible

The Hebrew Bible is a term used by biblical scholars outside of Judaism to refer to the Tanakh , a canonical collection of Jewish texts, and the common textual antecedent of the several canonical editions of the Christian Old Testament...

) and is part of the Christian Old Testament

Old Testament

The Old Testament, of which Christians hold different views, is a Christian term for the religious writings of ancient Israel held sacred and inspired by Christians which overlaps with the 24-book canon of the Masoretic Text of Judaism...

. The Book of Esther or the Megillah

Megillah

Megillah may refer to:Bible:*The Scroll of Esther , read on the Jewish holiday of Purim.*Megillat AntiochusRabbinic literature:*Tractate Megillah in the Talmud....

is the basis for the Jewish celebration of Purim

Purim

Purim is a Jewish holiday that commemorates the deliverance of the Jewish people in the ancient Persian Empire from destruction in the wake of a plot by Haman, a story recorded in the Biblical Book of Esther .Purim is celebrated annually according to the Hebrew calendar on the 14th...

. Its full text is read aloud twice during the celebration, in the evening and again the following morning.
The biblical Book of Esther is set in the third year of Ahasuerus

Ahasuerus

Ahasuerus is a name used several times in the Hebrew Bible, as well as related legends and Apocrypha. This name is applied in the Hebrew Scriptures to three rulers...

, a king of Persia.