What and When
Like Chanukah, Purim is a Rabbinic holiday in that it is not mentioned in the Five Books of Moses (although some opinions say that it is alluded to). It is, however, prophetically based, as its source, Megillat Esther, is part of the Biblical canon.
Because Purim is a Rabbinic holiday, there is no prohibition of doing creative work, as on Shabbat and Yom Tov. However, it is preferable not to go to work on the holiday.
Like all Jewish holidays, Purim begins at sunset. However, because it is a rabbinic holiday, there is no candle lighting.
Two different Purims: Purim and Shushan Purim
Unique to the Jewish calendar, Purim is actually observed on different days depending on the location in which it is celebrated.
The majority of the Jewish people celebrate Purim on the 14th of Adar.
Shushan and all cities that had walls at the time of Joshua’s conquest of Canaan celebrate Purim on the 15th of Adar.
A city’s population must celebrate on the day appropriate to its city.
The date for Purim for “cities without walls” is based on Esther 9:16-17.
“And the rest of the Jews in the states [not Shushan] of the king grouped together, protecting their lives, and were relieved of their enemies…on the 13th of the month of Adar, and they rested on the 14th, making it a day of feasting and joy.”
The date for Purim in “walled cities” is based on Esther 9:18.
“But the Jews in Shushan grouped together on the 13th and 14th, and rested on the 15th, making it a day of feasting and joy.”
While only the residents of Shushan rejoiced on the 15th, the rabbis decreed that all cities with walls at the time of the conquest share the latter date, in order that Jerusalem should also be separated out for honor.
How does this effect Purim today:
All modern walled cities celebrate on the 14th.
The only modern city that celebrates Shushan Purim is Jerusalem.
In order to extend the joyous celebration, many in Israel first celebrate outside of Jerusalem and then join the Shushan Purim celebrations in Jerusalem.
NOTE: Purim activities should be performed on the appropriate date for one’s city.