Siman 697

The first Adar is like Shevat. And if they read the four parshiyot in it—it is nothing, and they must return and read them in the second. For even according to the one who holds in the Gemara (6b) that post facto if they read them in the first—they have fulfilled their obligation, see there, this is when they did not know that the year would be intercalated, and that was during the time of the Temple. But now that we act according to a calculation, and the intercalation is known beforehand—they have not fulfilled their obligation. And only regarding eulogy and fasting—it is forbidden on the fourteenth and fifteenth of the first Adar. And some say that even regarding eulogy and fasting they are permitted, for that which was explained in the Gemara there that it is forbidden—this is also when they did not know of the intercalation. But the primary ruling follows the first opinion.

Therefore, it is forbidden to eulogize or fast on "Purim Katan," which is the fourteenth and fifteenth of the first Adar. And we do not perform nefilat apayim, and we do not say the psalm "May Hashem answer you on the day of distress." However, other matters are not practiced on them. And they are permitted for work. And some say that one must increase in feasting and joy on the fourteenth of the first Adar. But the custom is not so, for it implies explicitly in the Gemara there that there is no obligation at all in the first. Our teacher, the Rema, wrote: Nevertheless, one should increase somewhat in the meal in order to fulfill the obligation according to those who are stringent. And a good heart feasts always. Thus far his words, meaning: that the essence of the service of Hashem is with joy, and therefore when his intention is for the sake of a mitzvah—it is a feasting and joy of a mitzvah. And may Hashem gladden us with the building of the Beit HaMikdash, and with the coming of the righteous redeemer, Amen. With the help of Heaven, the laws of Megillah and all the laws of Shulchan Aruch Orach Chaim are concluded.