Question: The two Sundays prior to Christmas seem to have almost identical themes. What is the difference between the Sunday of the Holy Forefathers and the Sunday of the Holy Fathers (the Sunday immediately before Christmas)?
The themes of these two commemorations are indeed close. The Titles of these two commemorations are part of the confusion. The second Sunday before Nativity is called the Sunday of the Holy Forefathers, to distinguish it from other Sundays of the Holy Fathers (which commemorate the Fathers of the Ecumenical Councils) at other times during the year. The proper title of the Sunday before Nativity is actually "The Sunday Before Nativity", and only secondarily is it referred to as a commemoration of "the Holy Fathers" (Christopher Veniamin, trans. Saint Gregory Palamas: The Homilies (Waymart, PA: Mount Thabor Publishing, 2009), p. 632, note 952).
In the service of the Holy Forefathers itself, we hear the following hymns:
"O ye faithful, let us praise today all the fathers of the old Law: Abraham, the beloved of God, and Isaac, who was born according to the promise, and Jacob and the twelve patriarchs, the most meek David, and Daniel, the prophet of desires, glorifying with them the three youths that transformed the furnace into dew, and who ask remission of Christ God, Who is glorified in His saints" (The Doxasticon at Lord, I have cried...).
"Come, ye lovers of the feasts of the Church, and with psalms let us praise the assembly of the forefathers: Adam, the forefather of us all, Enoch, Noah, Melchizedek, Abraham, Isaac and Jacob; and, after the Law, Moses and Aaron, Joshua, Samuel and David, and, with them, Isaiah, Jeremiah, Ezekiel, Daniel and the twelve prophets, together with Elijah, Elisha and all the rest, Zachariah and the Forerunner; who all preached Christ, the Life and Resurrection of our race" (The Doxasticon of the Vespers Aposticha).
During the Canon, we also hear mention of Abel, Enoch, the Patriarch Joseph, and Job... none of whom were ancestors of Christ.
The prophets of God are mentioned by name: Hosea and Micah, Zephaniah and Habbakuk, Zechariah and Jonah, Haggai and Amos, Malachi, Obadiah and Nahum, Isaiah, Jeremiah and Ezekiel, together with Daniel, Elijah and Elisha -- and none of these were ancestors of Christ.
There is also mention of the Fore-mothers: "Hannah and Judith, Deborah and Huldah, Jael and Esther, Sarah and Miriam the sister of Moses, Rachel and Rebecca, and Ruth the exceeding wise" -- only 3 of whom are ancestors of Christ.
So the Sunday of the Holy Forefathers commemorates all of the faithful of the Old Testament, with special emphasis on Daniel and the Three Youths.
The Sunday before Nativity has many similar themes, and even some of the same hymns, but being the Sunday of the Genealogy, there is more of an emphasis on the Patriarch Abraham. He is mentioned about twice as frequently in this service. The Prophet Daniel is again included, but less frequently than in the previous Sunday. So there is a good deal of overlap, but there is a subtle shift from focusing on all the fathers and mothers of the Old Testament, to Christ's ancestors -- especially Abraham.
See also:
The Synaxarion of the Sunday of the Holy Forefathers
The Sunday Before Nativity, by Sergius V. Bulgakov.