The Magen Avraham (no. 8) adds that the reason for the custom not to make weddings during the Nine Days, even though it is a mitzvah to get married, is that it is an inauspicious time. This explanation of the custom seems inadequate because the Gemara (Yevamot 43a) states explicitly that the reason we do not make weddings in the beginning of the month of Av is because we reduce our joy. It seems difficult to reconcile the Magen Avraham’s explanation that the custom is due to the Nine Days being an inauspicious time with the view expressed in Yevamot that the reason is due to a required reduction in joy.
We can explain this difficulty through a more precise understanding of why preparation for a joyous occasion that constitutes a mitzvah is permitted during the Nine Days. We are permitted to prepare for a joyous occasion that constitutes a mitzvah not because such preparation is an exception to the general rule against preparation for joyous occasions. Such preparation is permitted, rather, because the general prohibition does not apply at all to preparation for a joyous occasion that constitutes a mitzvah. This is because the nature of the general prohibition is that one is not permitted to prepare for an event that is primarily a joyous occasion. When the joyous event is also a mitzvah, the preparation is viewed as for the mitzvah, not for the component of joy that might accompany it. The preparation is permitted because it does not fall within the general prohibition of preparing “for an event that is primarily a joyous occasion.” This preparation, rather, is for a mitzvah occasion.
However, while a wedding is a mitzvah, the essence of the mitzvah itself is to bring joy to the bride and groom. Therefore, even though preparation for mitzvah occasions does not generally fall under the general prohibition of preparing for joyous occasions during the Nine Days, preparation for a wedding does fall under the general prohibition because the mitzvah is for the object of the preparation, the wedding, to be a joyous occasion.
Adapted from Shiurei Harav on Mourning and Tisha B’Av, based on the lectures of Rabbi Joseph B. Soloveitchik. This and other books are available for purchase at www.ou.org/sefarim