The Sages said, "When Av arrives we curtail [our] joy, and when Adar arrives we increase [our] joy" (Ta’anit 29a). In saying this, the Sages teach us that nothing happens by chance and that every season has its own character and nature. If both Temples were destroyed on the ninth of Av, it is a sign that the beginning of Av is naturally predisposed to calamity. And if the miracle of Purim occurred in the middle of Adar, it is a sign that this month has the capacity to transform bad into good. We feel ordinary joy over the good that exists in the world, but our joy is not complete, because there is still evil and pain in the world. However, when even the bad turns into good, our joy increases and becomes complete. This is what happened on Purim: God turned the bad into good and saved His people, Israel. This teaches us that everything that happens in the world – even the bad – will eventually become good. The greater our faith and our commitment to Torah, the closer we bring the ultimate redemption, when all evil will turn into good, and when joy will abound in the world. Since the month of Adar has the unique character of transforming bad into good, "when Adar arrives we increase [our] joy."
The Sages further recommend (ibid. 29b) that a Jew who has a legal dispute or business deal to settle with a non-Jew should avoid him during the month of Av, because that month portends misfortune. Instead, he should try to make himself available for litigation or a business deal during Adar, when the Jewish people experiences good fortune.