What's the source
for the Jewish custom to say Kaddish (the Mourner's Prayer), and why is it said for 11 months?
The source for saying Kaddish, the Mourner's
Prayer, for a period of 11 months is found in the Mishnah (Ediyos 2:10; also Shabbos 33b):
"There are 5 things that endure 12 months... [One of them is] the judgment of the guilty
in Gehinnom [Purgatory]."
By implication, if one is NOT guilty the judgment is finished no later than by the
conclusion of the 11th month.
Sources indicate that the vast, vast majority are cleansed of their sins within the 12 month period
and are thus considered righteous [Derech Hashem 2:2:4]. And that's why in halacha the assumption is that no
matter how the parent lived their life one still says Kaddish for 11 full months and no more.