Q. Why was the seventh of Adar or Yohrzait of Moshe Rabbenu chosen as the day for fasting and the following a party or Seuda of the Chevra Kaddisha?
A. It is a tradition for many such societies to dedicate the seventh of Adar as a day devoted to honor steadfast volunteers (and their families) for their dedication and personal sacrifice for the holy work they do. This banquet is customarily held at the conclusion of the seventh day of Adar, which tradition teaches is the anniversary of the death of Moshe Rabbenu.
A number of reasons are given in Seforim why this day was chosen.
Most prominent is that Moshe Rabbenu is remembered as the singular human being that did not require the services of a Chevra Kaddisha, since as the Torah describes, Hashem himself tended to his last needs. The members of a Chevra Kadisha, which are usually not remunerated, would be happier if their work was also not needed.
Horav Chanoch Ehrentreu Shlit’a added that it so special to do something that Hashem Himself did.
Another reason could be that in history usually the ones taking care of the deceased were their children or other family members. On the Torah record, it seems to be that the first one to be taking care of someone not directly related to him was indeed Moshe Rabbenu, as described in the beginning of Parshas Beshalach (13: 9): “Moshe took Yosef’s bones with him.” As described by our Sages, that was done at a great financial loss for him.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a