“”Mordechai: You'll be a hero, everyone will admire you, people each year will dress up as you!
Esther: So they'll dress up like whores.
Mordechai: Only in high school.
|—HaYehudim Baim (The Jews Are Coming)|
Purim is a Jewish holiday that celebrates an ancient plot to kill the Persian Jews perpetrated by a court vizier called Haman (an Agagite, not a Persian) and thwarted by Esther, wife of the Persian King, Xerxes. It is celebrated by the Jews today because the Jews won and Haman was impaled on a giant pole! It also involved a lot of BS involving court etiquette and the King's inability to perceive the ethnic bickering in his sprawling, multicultural kingdom. The Book of Esther is read in synagogue to recount the story.
But the most important thing is that it's a mitzvah to get drunk! Yes, this is when we all drink till we don't know the difference between
blessed-be-Conservapedia and cursed-be-Rationalwiki blessed-be-Rationalwiki and cursed-be-Conservapedia (learned Jews will understand that line).
Other mitzvahs of the day include giving gifts of food to your friends and giving charity to the destitute. The most important mitzvah of the day is having a big party!
It is also common to dress up for Purim. This is more like an old-fashioned masquerade ball rather than Halloween. Many people choose to dress up as characters from the Purim story, but others do not.
This holiday goes along the theme of many Jewish holidays: "They tried to kill us, but we killed them instead! So let's eat!!"; the difference on Purim is that it's custom to get blitzed, too.