The wackiest of the many holidays on the Jewish calendar is Purim, which falls this year on Saturday evening and Sunday (and a day later in Jerusalem). Purim commemorates the Jews’ deliverance from a genocidal decree of the Persian Empire sometime in the 5th century BCE. Its story is told in the Book of Esther, the last of the 24 books of the Hebrew Bible to be canonized.
Purim, as laid down in the ninth chapter of Esther two and a half millennia ago, is a joyous day, marked by a festive meal, the sending of food gifts, the giving of charity, and the public reading of Esther (mostly in synagogues, though in Israel you can tune into synagogue readings on TV). The Talmud even tells you to get drunk on Purim until you can’t tell the difference between “cursed be Haman” (the villain of the Book of Esther, who is eventually hanged) and “blessed be Mordechai” (a good guy, who eventually becomes the king’s second-in-command).
At some point in medieval times Purim also became a costume holiday. While, in today’s Diaspora, Purim is mostly celebrated by observant Jews, in today’s Israel it’s a countrywide event and you can see colorful, often bizarre costumes everywhere, along with carnival processions on city streets (a custom begun in a then brand-new city, Tel Aviv, in 1912).