Responsa zu Schemuel II 21:1

From these words of theirs, we may learn that the good deeds of the son, when they are done in the congregation of God in the synagogue to benefit the community to bless [God] with the congregation, by saying to them, Barchu, raises up the soul of their fathers from the punishment of Geihinnom. And from this, we may learn that the same is true, and for the same reason, with the saying of kaddish, since he benefits the community, as he says, "May [His name] be magnified [Yitgadal] and sanctified, etc. May His great name, etc. and say, Amen." With this beneficial deed, he saves his father from he punishment of Geihinnom. However, regarding the kaddish that we say today, it was not known to our rabbis, the Geonim, may their memory be blessed. And likewise was it said in Teshuvot HeGeonim, Coronel, 94: And you asked [about] someone who died, and after a year, his relatives bring his bones [for reburial], whether we say, tzidduk hadin and kaddish or not. You should know that we have not directly found [the saying of] kaddish, which is Yitgadal, over the dead, but only after the eulogy or after tzidduk hadin [said at the burial]. As any time there are words of Torah, we answer after them, "Amen, May His great name, etc." And it is the same with a eulogy or tzidduk hadin, etc. However, this thing is within twelve months, but not after twelve months. Rather, that which we say, And the Lord said, "It is for Saul, and for his bloody house" (II Samuel 21:1) — for Saul because he was not eulogized properly, etc., we then say that David said, "[About] Saul, his twelve months of the year have already passed and it is not the proper way to eulogize." (Yevamot 78b-79a) We learned from it, that it is not fit to eulogize after twelve months. And since there is no eulogy and no tzidduk hadin, it is not the way to [say] kaddish. (See Otzar Geonim of Dr. B. M. Levin, Mossad HaRav Kook, p 25, 139.)