The Gemorah says in Taanis 29a: “משנכנס אדר מרבין בשמחה”.
What does this mean exactly? What should one do to “marbin” in Simcha? What’s an example?
The Rambam and the Shulchan Aruch make no mention of the joy of Adar.
The Magen Avraham (686:5), however, and in his wake the Mishnah Berurah(686:8), rule that “when the month of Adar enters, we increase our joy, and one who has a dispute with a non-Jew should settle it in Adar.” This is also ruled by theKitzur Shulchan Aruch (141:1) and by the Aruch Hashulchan (686:6).
Beyond the idea of when to settle disputes with non-Jews, these authorities don’t mention any application of the joy of Adar.
However, there are sources that note expressions of joy for Adar, as cited below in the Sources.
The Gemara does not explain how we are to augment our joy in the month of Adar.
It is noteworthy that the Meiri (Taanis, loc. cit.) interprets the directive of the Gemara in a novel manner, explaining, “It is proper to pray and to express gratitude to Hashem at all times, according to that which occurred at that time. One must bless upon the good and upon the bad, as we have explained.” Clearly, in the opinion of the Meiri there is no specific expression of the joy of Adar.
At the other extreme, the Nimmukei Orach Chaim (686) writes that during the month of Adar each person should augment his joy in the manner that suits him personally, thereby fulfilling the directive of Chazal. This will apparently apply to food, drink, and other matters, each according to his disposition. Indeed, it is known that Rav Chaim Shmuelevitz used to increase his hours of Torah study over the month of Adar: This was his way of augmenting joy!
However, authorities in general make no mention of this concept, and it appears that there is indeed no concrete manner in which a person must augment the joy of Adar, beyond ensuring a general atmosphere of simchah as appropriate for the special month.