The Torah interrupts the telling of the tragic episode of the sale of Joseph in order to relate the episode involving Judah and Tamar. The Sages emphasize that the flourishing of the messiah’s glory began from this incident, as King David, progenitor of the messiah, was a descendant of Peretz, son of Judah and Tamar.

The tribes were involved in the sale of Joseph; Joseph was involved in his sackcloth and in his fasting over his sorrow at having been sold, Reuben was involved in his sackcloth and in his fasting to repent for his sin (see Genesis 35:22), Jacob was engaged in his sackcloth and in his fasting in mourning over the loss of Joseph, Judah was involved in taking a wife for himself, and the Holy One, blessed be He, was involved in creating the light of the Messianic king descended from Judah and Tamar…. This tells us that before the first enslaver was born, the progenitor of the ultimate redeemer was born. (Bereshit Rabba 85)