As we all know, “Mishenichnas Adar Marbin B’Simcha – when one enters into Adar, we increase our joy” (Taanis 29a).  There is a discussion in the Achronim as to whether this increased level of joy commences with the first day of Adar I. The Sefer Adar U’Purim by HaRav Yoel Schwartz,

The Chofetz Chaim writes that there are only two Jewish festivals during the winter: Chanukah and Purim. At Chanukah, the enemies of the Jewish people tried to take away from us, G-d forbid, our holy Torah, which is our soul and spirit. At Purim they tried to destroy us physically, G-d forbid, by attacking our bodies. What was the result? We were victorious, and we retained