The Sages did not originally establish Purim as a holiday on which work is prohibited. Over time, however, the Jewish people developed a custom to refrain from work on Purim, out of respect for the sanctity of the day, and the custom is binding. The Sages go so far as to say that anyone who works on Purim will never see any blessing from it (Beit Yosef, SA, Rema 696:1).
Therefore, one may not go about his regular work on Purim. If, however, refraining from work would cause him or his employer a great loss, he may work on Purim. In addition, a poor person who has nothing to eat may work on Purim (SHT 696:2-3).
One may do joyous work on Purim, like preparing a new home for a son who will soon be married and planting decorative trees in one’s yard. In addition, one may do mitzva-related work, like writing down novel Torah insights. One may also do easy work, like writing a letter. All of this is permitted provided that it does not ruin one’s joy or prevent one from performing the mitzvot of Purim (SA 696:1, MB ad loc. 6).
One may cut one’s nails on Purim, because this is an easy task, while one may not wash clothes by hand, sew, or cut one’s hair (Ben Ish Ĥai, Hilkhot Purim 21 [Tetzaveh]). However, if they are needed on Purim, one may do arduous tasks, including washing clothes by hand, sewing, and cutting one’s hair (Rema 696:1).
Technically, one may engage in commerce on Purim, because successful transactions can bring one joy. However, it is proper to be stringent in this matter, because such endeavors can drag on and take away from the joy of Purim. Therefore, it is proper to open only stores that sell Purim necessities (MB 696:3, AHS 696:2, Kaf Ha-ĥayim 696:5).
The custom of refraining from work on Purim takes effect only during the day. It is true that some Aĥaronim rule stringently and prohibit work at night, as well. However, the very fact that there is a debate regarding working at night shows clearly that there is no accepted custom prohibiting work at night. Therefore, one may work on the night of Purim (see BHL 696:1).
Residents of unwalled cities may work on the Purim observed by residents of walled cities, and residents of walled cities may work on the Purim observed by residents of unwalled cities (SA 696:2).