Doctrine & Covenants 110
This is the section that records the marvelous vision Joseph Smith and Oliver Cowdery witnessed after the Kirtland Temple Dedication. I always find it fascinating that the Lord timed it so that Elijah came to bestow the sealing power of the priesthood on the exact day that Jews set a place for him at their Passover Seder, anxiously awaiting his arrival to herald the coming of the Messiah. Elijah did come in 1836 to fulfill that prophecy from Malachi, but he came to the Kirtland Temple.
Another thing that helps me understand this section a little bit better is to correlate it to the three-fold mission of the church. The "gathering of Israel" is "proclaim the gospel," the "gospel of Abraham" relates to his patriarchal responsibility to strengthen families and is "perfect the Saints," and Elijah's "turning the hearts of the fathers to the children" is "redeem the dead."