Purim Katan (14 Adar I)

[If they] read the Megillah during the first Adar and then the year was intercalated, one reads during the second Adar.
The only difference between the first Adar and the second Adar is the reading of the Megillah and gifts to the poor.

It was taught: If they read the Megillah during the first Adar and the year was then intercalated, they read it during the second Adar, as all mitzvot that are practiced during the second Adar are practiced in the first Adar, except for the reading of the Megillah.

Rabbi Eliezer, son of Rabbi Yosei, says: "They do not read it during the second Adar, as all mitzvot that are practiced during the second are practiced during the first."
Rabban Shimon ben Gamliel says from the name of Rabbi Yosei: "They even read it during the second Adar, as all mitzvot that are practiced during the second are not practiced during the first. And they all agree with regard to eulogy and with regard to fasting that they are prohibited on this and on that."

Rabbi Ḥiyya bar Avin said: "Rabbi Yoḥanan said: 'The halakha is in accordance with Rabban Shimon ben Gamliel, who said from the name of Rabbi Yosei.'"

And Rabbi Eliezer beRebbi Yose holds, even the reading of the scroll (at the outset on the first: And it appears) that he made a precise inference from that which it taught an extra [teaching in the] mishnah: that all commandments that are practiced in the second are practiced in the first, (which implies) at the outset (lekhatchila). And there are some that are accustomed to making days of drinking-parties and joy on the fourteenth and fifteenth of the first Adar, and the quick [reading] of our mishnah also implies this, as it says [that there is no difference between the two months] except for the scroll and gifts to the poor. [From this] it is implied that concerning the matter of drinking-parties and joy, this and that (both months) are the same. And [this] is not lucid; as behold, we say in the gemara, "Concerning the matter of eulogies and fasts, this and that (both months) are the same." [From this] it is implied that feasting and joy are not [treated the same in both months]. As per force, one is not dependent upon the other; as if one were dependent upon the other, let [the gemara] make us understand that drinking-parties and joy are practiced on [the fourteenth and fifteenth of the first Adar], and automatically, it would be forbidden for eulogies. As behold, the days that are mentioned in Megillat Taanit that are forbidden for eulogies [also] do not have drinking-parties and joy (so we see that drinking-parties implies not having eulogies, even if having no eulogies does not imply drinking-parties). And so is the law (halakha), that there is no need to be stringent to make drinking-parties and joy in the first Adar.

Rabbi Yitzḥak ben Jacob Alfasi, of blessed memory (1013–1103), wrote that one needs to increase one's meal on the 14th of Adar 1, but he wrote that one does not need to increase one's meal on the 15th of Adar 1. And my father, of blessed memory (Rabbi Asher ben Yeḥiel (1250-1327)) wrote according to the initial opinion.

... and he wrote that the matter is close also that it is appropriate to increase one's feasting on the 14th of the first Adar, but for the purpose of sending portions since it was comparable to sending gifts to the poor, which is only in the second Adar, this implies that even sending portions is only for the second Adar.

And Tosafot wrote "And there are some that are accustomed to making days of drinking-parties and enjoying themselves on the fourteenth and fifteenth of the first Adar, and the quick [reading] of our mishnah also implies this..., but [this] is not lucid; as behold, it says in the Talmud, 'Concerning the matter of eulogies and fasts, this and that (both months) are the same.' [From this] it is implied that feasting and joy are not [treated the same in both months].... And so the rule is that there is no need to be stringent to make drinking-parties and joy in the first Adar."
Rabbi Meir HaKohen (end of the 13th century) wrote [in his] Hagahot Maimonit, quoting Rabbi Yitzḥak ben Yoseph of Corbeil (d. 1280)'s Sefer Mitzvot Katan, "That throughout the world, people are generally accustomed to making drinking-parties nor merriment only on 14 Adar I, even though people are accustomed to making merriment and drinking-parties on both 14 and 15 Adar II. And the reason is on account of the Talmudic statement that there is no difference between 14 of Adar...and it doesn't mention 15 Adar I. And perhaps it mentioned it, since 14 Adar is the main date, but regarding doing labor, there is no prohibition of doing so on the first Adar, unless it is a locale where they are accustomed."
But not, we are accustomed to increase our feasting on neither the 14th nor 15th of the first Adar. Nevertheless, we are accustomed to not fall on our faces nor to say Psalm 20 since they are days of miracles and saving.

On the 14th and the 15th day of Adar I, we do not fall upon our faces [recite Tahanun], and we do not say [Psalm 20] "A Psalm... may the Lord answer you," and on those [days], it is forbidden to eulogize or to fast. There are those who say that even eulogy and fasting are permitted.

There are those who say that one is obligated to increase in drinking and joy on the 14th of Adar I (Rabbi Ya'akov ben Asher (1270-1340) quoting Rabbi Yitzḥak ben Jacob Alfasi (1013–1103)).

But, we are not accustomed this way.
Nevertheless, one should increase their feasting slightly, in order to fulfill the opinion of those who are stringent (Rabbi Meir HaKohen (end of the 13th century) quoting Rabbi Yitzḥak ben Yoseph of Corbeil (d. 1280)).
And "the good-hearted has a continual drinking-party" (Proverbs 15:15).

Rabbi Shimson ben Ẓadok (d. 1312) wrote that one should increase (Tashbetz Commentary §178), and Rabbi Yeḥiel of Paris (d. 1268) was accustomed to increase and to invite people, according to the straight-forward understanding of the Mishnah: "The only difference between Adar I and Adar II is the reading of the megillah and gifts to the poor" (Megillah 1:4). And it is good how Rabbi Moses Isserles (1530-1572) concludes here: "And a good-minded person always enjoys drinking-parties."

It's inferable even on the 15th [of Adar I], and it's similarly inferable in [Rabbi Abraham Klausner (2nd half of 14th c.)'s] Customs Book, but the language of Rabbi Moses Isserles (1530-1572) "and, nevertheless, one is obligated to increase on the 14th..." seems to infer not on the 15th. And it's possible that it's included with how he concludes "And a good-minded person always enjoys drinking-parties."

It's possible to say that, according to what Rabbi Isserles wrote earlier in §670 in the gloss on section 2 regarding Hanukah, that "there are those who say that there is a bit of mitzvah with increasing feasting...and we are accustomed to sing songs and praises at a feast where we are increasing with them, which would then be a mitzvah meal." See there. And from there, we can learn that we should increase a little bit with feasting for 14 Adar I in order to fulfill the requirement for those who consider this matter stringently that there is to accustom oneself to sing songs and praises, which would then be a mitzvah meal. And this is hinted at with Rabbi Isserles' conclusion "and a good-minded person will always be drinking-partying". And this is according to what [Rav Yehudah quotes Shmuel as] saying in the Talmud: "Whence is it that song is a fundamental accompaniment from the Torah?" and [Rav Matna's saying that] "which service that is joyful and good-minded - this is song" (Arakhin 11a).

Therefore, it is forbidden to eulogize or fast on Purim Katan, which is 14 Adar I and 15 Adar I. And we do not fall on our faces nor do we say Psalm 20, but we are not accustomed to perform other practices, but work is permissible.
And some say that it is necessary to increase drinking-parties and joyfulness on 14 Adar I, although our practice is not as such, since it is explicitly inferable from the Talmud there that there is no obligation in the first Adar. Rabbi Moshe Isserles wrote: "Nevertheless, one should slightly increase one's feasting in order to fulfill the requirement according to those who consider this matter stringently. And a good-minded person is always drinking", which is to say that the primary serving of God is joyously. And, therefore, his intentionality is for the purpose of mitzvah-performance, which is the drinking-partying and enjoyment of mitzvot.

Rabbi Shimson ben Ẓadok (d. 1312) wrote that there is to increase, and Rabbi Yeḥiel of Paris (d. 1268) was accustomed to increase and to invite people. And this is how Rabbi Moses Isserles (1530-1572) concluded.

"And a good-minded person is continually enjoying drinking-parties" is that it is good to increase one's drinking-parties out of honor of the miracle that occurred during these times.