Even though the holiday of Purim is not one of the God-ordained holy days as commanded by God to Moses, it is observed by Jews today on the 14th day of Adar, which is usually in March. The 13th of Adar is the day that Haman chose for the extermination of the Jews, and the day that the Jews battled their enemies for their lives. 

On the day afterwards, the 14th, they celebrated their survival. In cities that were walled in the time of Joshua, Purim is celebrated on the 15th of the month, because the book of Esther says that in Shushan (a walled city), deliverance from the massacre was not complete until the next day, the 15th.

The Purim holiday is preceded by a minor fast, the Fast of Esther, which commemorates Esther's three days of fasting in preparation for her meeting with the king. 

Those who celebrate Purim send out gifts of food or drink, and make gifts to charity. The sending of gifts of food and drink is referred to as shalach manos (lit. sending out portions). 

It is customary to hold carnival-like celebrations on Purim, to perform plays and parodies, and to hold beauty contests. 

For some additional insights into the customs of the day in Esther’s time we would recommend this book, The Gilded Chamber: A Novel of Queen Esther by Rebecca Kohn.