Why is it not mentioned in Shulchan Aruch the important Halacha of "Mishenichnas Adar Marbim Besimcha", that from the beginning of the Month of Adar, one has to increase joy and happiness?
A. Poskim mention that the reason is since in essence the whole month of Adar could be on special occasions a time for reading the Megila including Rosh Chodesh, as mentioned in Shulchan Aruch (O.H. 688: 7). The Rema (ibid.) explains that we follow this tradition, so it is as is, a time for joy. (see Chasam Sofer O.H. 160).
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