The first [Adar in a Jewish leap year] - However, everyone agrees that we don't say [the additional paragraphs of] "Al Hanisim" [in those days' prayers] because the Megillah is not read [until the following month]. Nevertheless, if one says [these paragraphs], they do not need to go back [and repeat the Amidah].

May [the Lord] answer you - And we don't say אל ארך אפים on both days of Purim Katan, since the Psalm of למנצח יענך (Psalm 20) and the prayer אל ארך אפים are equal [thus, if one is omitted, the other is as well]. Regarding the recitation of מזמור לאסף (Psalm 82) on Purim Katan, there are places in Levov where they omit it on days where Tachanun is not said. [However,] there are places where it is said unless Hallel is recited that day, such as in Frankfurt (Oder). If Purim Katan falls out on Shabbat, we do not say צדקתך [after the Amidah during Shabbat Mincha]. If one experienced a miracle in Adar, and took upon himself to always make a feast [on its anniversary], if it [the miracle] occurred in a non-leap year, he should make his feast during Adar I in a leap year. If it occurred during Adar II of a leap year, he should make his feast during Adar II. This feast that is made because of a miracle is [considered] a Seudat Mitzva, since any feast made to commemorate the wonders of Hashem is a Seudat Mitzva.

And fasting - However, the agreed upon custom is that mourning is permitted during Adar I. If one explicitly vowed to fast on Purim Katan, some say that vow takes effect, since it is only a custom [to prohibit fasting]. However, if one simply vowed to fast on every Monday and Thursday of the year, it is possible to say that they would have excluded Purim Katan in their mind [as they were making their vow].

On the 14th - But not on the 15th, even for walled cities.

Nevertheless, one should increase [slightly] - It is written in the glosses of the Tashbatz that one should increase [in feasting], and Rabbeinu Yechiel of Paris would regularly invite many guests to partake in his increased [feasting / celebration]. And this is why the Rema concludes with "A good heart will always feast" (Proverbs 15:15) – meaning that it is good to increase [in celebration] in honor of the miracle that occurred at these times.