The Gemara in Masechet Ta’anit (29a) tells us, “Rabbi Yehuda son of Rav Shmuel ben Shilat taught in the name of Rav: Just as when the month of Av begins we diminish our happiness, so too, when Adar begins we increase our happiness. Rav Papa says, therefore, if a Jew has a court case with a non-Jew pending, he should avoid having it during the month of Av when the Jewish nation’s fortune is bad and try to have it held during the month of Adar when the Jewish nation’s fortune is good.”
The source for this is based on a verse in Megillat Esther which states, “And the month which was switched for them from tragedy to joy,” which teaches us that the good fortune of this month brings about salvation and goodness for Israel since their fortune is optimal during this month.
Nevertheless, Hagaon Chatam Sofer (Chapter 160) points out the distinction in the words of Maran Ha’Shulchan Aruch in that whereas in the laws of Tisha Be’av (Chapter 551), Maran quotes the words of the Sages that when Av begins, we decrease our joy, Maran does not quote the corresponding teaching that when the month of Adar begins, we increase our joy. Furthermore, the Rambam does not quote this teaching of our Sages at all.
The Chatam Sofer explain that it is possible that the reason why the Rambam and Shulchan Aruch omitted this law of rejoicing during Adar is because Rav Papa who maintained that one should try to schedule a court case with a non-Jew to be held specifically during the month of Adar is because he is of the opinion that the Jewish nation is subject to the influence of the constellation and during Adar when their fortune is good, this is cause for celebration. On the other hand, we maintain that the Jewish nation is not subject to the influence of the stars (based on the Gemara, end of Shabbat) and thus, the Rambam and Maran did not mention this law regarding the month of Adar at all.
However, many Poskim defend the ruling of Rav Papa that when Adar begins, our joy increases; some actually post animated signs during the month of Adar to that effect. Although we maintain that the Jewish nation is not subject to the influence of the constellations, there is still room to rejoice during these days because this entire month has been switched from mourning to rejoicing in the times of our forefathers. The reason why Maran omits this law is because he had already written (in Chapter 688) that some rule that one may read the Megillah from the beginning of the month of Adar. Thus, the entire month of Adar is certainly a time for rejoicing. The Sefer Hitorerut Teshuva (Chapter 473) resolves this issue likewise. In previous years, we have explained this matter further based on the words of the Ritba and as discussed in the works of Maran zt”l.
The great Rishon Le’Zion, Hagaon Rabbeinu Yitzchak Yosef Shlit”a, explains (in his Yalkut Yosef- Purim, page 192) that the reason for rejoicing during the month of Adar is out of thanksgiving to Hashem for all the miracles he performed for us during this period. It is also an indication that these days are auspicious for goodness and blessings.
He adds that just as during the month of Av, we progressively decrease our joy until the epitome of mourning on the Ninth of Av, during the month of Adar, we progressively increase our joy in preparation for the most joyous day of the month, the day of Purim.
He quotes the Sefer Sefat Emet (on Ta’anit ibid.) who explains that it is possible that since during the month of Adar during the eras of the Mishkan (Tabernacle) and the first and second Bet Hamikdash the Half-Shekel coins were collected in the Bet Hamikdash and the Jewish nation donated them joyously, there was a spirit of great joy in the world. Until this very day, from the time Parashat Shekalim is read in the synagogue (on the Shabbat of or preceding Rosh Chodesh Adar), a great joy is awakened in the world.