The Mishna in Masechet Megila (6b) establishes that in a leap year, which contains an extra month of Adar, the festival of Purim (which is normally celebrated in Adar) is observed during Adar Sheni (the "second Adar"). This means that all the Misvot associated with Purim – reading the Megila, sending Mishlo'ah Manot, giving money to the poor, and the festive meal – apply specifically on the fourteenth of Adar Sheni during a leap year.
Nevertheless, it is customary to observe the fourteenth and fifteenth days of Adar Rishon as "Purim Katan" – literally, "minor Purim." (This year 5768/2008, a leap year, Purim Katan falls out on Wednesday Feb. 20th and Thursday Feb. 21st.) On these days we omit the Tahanunim section of the prayer service, including the Viduyim (confessions). We also omit the paragraph of "La'menase'ah" and "Tefila Le'David" which are incongruous with the festive nature of these days. Tahanunim is likewise omitted from Minha on the afternoon of the thirteenth of Adar Rishon. When the fifteenth of Adar Rishon falls on Shabbat, we do not recite "Sidkatecha" during Minha. (The fourteenth of Adar never falls on Shabbat, but the fifteenth can occur on Shabbat.) It is forbidden to observe a fast on Purim Katan, even in observance of the Yartzeit of a parent, and even for Ashkenazic brides and grooms, who otherwise have the custom to fast on the day of their wedding. (This ruling is codified in the work Nimuke Orah Haim, 697.) Eulogies are also forbidden on Purim Katan, with the exception of eulogies delivered at the funeral of a Torah scholar.
Going to work is permissible on Purim day itself, so it goes without saying that one may perform such activity on Purim Katan, as well.
According to some authorities, it is proper to observe Purim Katan with some festivity and joyous celebration.
